Washington (CNN) A US border patrol marine unit intercepted a small speed boat near the southern coast of Puerto Rico last month -- an operation that led to the arrest of two foreign nationals after agents uncovered 1,320 pounds of cocaine, worth an estimated $17 million, on board.

On the surface, it seemed like a success story for federal and local law enforcement groups tasked with carrying out one of President Donald Trump's major policy priorities -- protecting the US border from drugs and criminals.

But it was also a sign that drug smugglers continue to use known waterway routes and transit zones to move illegal substances into the US despite Trump's emphasis on border security.

This unabated challenge has amplified questions around the President's call to allocate major funding toward a wall along the southern US border -- a project that is estimated to cost between $12 billion and $22 billion

Specifically, two major issues concerning the wall and drug smuggling have been raised: Will it actually reduce the flow of illicit substances into the US? And will money used to pay for its construction siphon off needed resources from other drug enforcement arms?

Based on the results of a presidential election that embraced the chant "build that wall!," Trump supporters have made their priority known.

White House press secretary Sean Spicer reiterated Trump's dedication to building the wall several times last week despite the project being left out of a spending bill to fund the government through September.

"Make no mistake, the wall is going to be built," and the President "wants it done as soon as possible," Spicer told reporters on Monday.

But evidence suggesting that drug traffickers will continue to find ways to move their products into the US even if a wall is eventually constructed raises a broader point about Trump's immigration approach and funding priorities.

Trump has managed to make a palpable difference in US immigration enforcement through sharp rhetoric and executive actions that, according to analysts, has at least partially contributed to a sharp drop in apprehensions of individuals illegally crossing the southwest border in both February and March.

There were roughly 12,000 total apprehensions at the southwest border in March, according to numbers obtained by CNN . That represented a 35% drop from this February and a 63% drop from March 2016.

Yearly drug seizures at the US-Mexico border have also declined slightly since January 2017, according to data provided by the Department of Homeland Security, and the White House has often touted similar trends to back Trump's promise that a wall "will get built and help stop drugs (and) human trafficking."

But law enforcement officials and experts told CNN that building a wall wouldn't solve the problem of illegal drugs entering the US, just displace it -- a point of view that the Department of Homeland Security recently acknowledged.

"Down doesn't mean stopped," David Lapan, a DHS spokesman, told reporters last week. "Part of the reason that drugs come through waterways is because they're looking for other ways around where we have good defenses."

Photos: Views from the border The US-Mexico border spans 1,933 miles. It runs from the Pacific Ocean to the Gulf of Mexico, across California, Arizona, New Mexico and Texas. President Trump's pledge to build a wall along the border was a key part of his campaign. He said this month that early construction of the wall would be funded with US tax dollars in order to get started quickly, and promised that Mexico "will reimburse us." Click through this gallery to see scenes from the border and perspectives from those who live and work near it. Hide Caption 1 of 20 Photos: Views from the border Pamela Taylor, 88, has lived in Brownsville, Texas, since 1947. Taylor says she has had people from "all over the world" arrive on her property, and she says she has even found them in her living room. Every night, she fills a cooler in front of her house with bottles of water for migrants who made the journey, Border Patrol officers, or anybody else who finds themselves near the front steps of her house. Hide Caption 2 of 20 Photos: Views from the border Taylor put this sign up down the road from her house during the 2016 presidential election. She says that the fence the US government put up near her property in 2007 doesn't work. "I would like for Mr. Trump, I would even feed him, if he will come down here and talk to the people," Taylor told CNN. "He is doing exactly what the government did to us in the beginning. He's not asking how it's going to affect the people that live here." Hide Caption 3 of 20 Photos: Views from the border The Rio Grande forms a border between Texas and Mexico. Much of the river straddles remote desert and farmland, such as this stretch in Progreso Lakes, Texas. Hide Caption 4 of 20 Photos: Views from the border On October, 26, 2006, President George W. Bush signed the Secure Fence Act, and said, "This bill I'm about to sign is an important step in our nation's efforts to secure our border and reform our immigration system." More than 1,250 miles of the border are in Texas, but the state only has about 100 miles of man-made borders. This fence line in Progreso Lakes, Texas, comes to an end, and leaves miles of border land open. Hide Caption 5 of 20 Photos: Views from the border Greg Henington, owner of Far Flung Outdoor Center in Terlingua, Texas, says he doesn't believe a wall is necessary. "The wall is not going to make a difference one way or another. It's just going to cost a ton of money and look dumb." Hide Caption 6 of 20 Photos: Views from the border In 1935, the federal government passed legislation that would enable the state of Texas to acquire the land that would become Big Bend National Park. The park, known for its remote beauty, shares the border with Mexico for 118 miles and is separated by the Rio Grande. Hide Caption 7 of 20 Photos: Views from the border Lilia Falcon owns Jose Falcon's restaurant in Boquillas del Carmen, Mexico, across the Rio Grande from Big Bend National Park in Texas. The border crossing closed after September 11, 2001, and reopened in April 2013. "This town just went dead," Falcon told CNN. "We are very happy that the border is reopened again. We feel very safe here, even tourists that come over here, the word is spreading out more that its very safe to come here." Hide Caption 8 of 20 Photos: Views from the border Many Texas residents in the Big Bend National Park region believe that the various miles of remote canyons and rivers already act as an "effective barrier" with Mexico. Hide Caption 9 of 20 Photos: Views from the border Robert Cameron, owner of Texas Border Tours in Progreso Lakes, Texas, is in favor of President Trump's proposed wall, but he knows it will be complicated to build through parts with rough terrain. "I want to see a wall," Cameron told CNN. "Not a fence. I want to see a wall. I want to see something that you can't see through, that you can't climb through." Hide Caption 10 of 20 Photos: Views from the border The scenic mountains, canyons and desert that make up the Big Bend region on the west Texas-Mexico border span an elevation of less than 1,800 feet near the Rio Grande to almost 8,000 feet in the Chisos Mountains. Hide Caption 11 of 20 Photos: Views from the border Michael Ryan has worked as a ranger at Big Bend National Park since 1999, and 10 years as a river guide in the region previously. "It's not just one border, it changes depending on where you are," he told CNN. Hide Caption 12 of 20 Photos: Views from the border The Boquillas Crossing, a one-of-a kind port of entry where you can take a small ferry boat across the Rio Grande and into the tiny Mexican village, reopened on April 10, 2013, following federal closure for more than a decade after the September 11, 2001, attacks. Hide Caption 13 of 20 Photos: Views from the border Marcos Paredes has lived near Terlingua, Texas -- a former "ghost town" -- for much of his life. He spent years as a law enforcement officer responsible for patrolling the Rio Grande. "To come up with a one-sized fits all solution for illegal immigration is crazy," Paredes told CNN. "Because the border is so different along its entire links." Hide Caption 14 of 20 Photos: Views from the border Alicia Bon Martin, born in Nogales, Mexico, and her husband, Chris Martin, work and live on both sides of the Arizona-Mexico border. They own organic farms in Mexico, a produce distributorship, and the restaurant La Roca, a Nogales, Sonora, institution started by Alicia's uncle and celebrating its 45th year anniversary this year. "We are lucky because we have the good things of both sides," Alicia told CNN. "I see this border as a much calmer than anybody portrays it to be." Hide Caption 15 of 20 Photos: Views from the border Tim Foley founded The Arizona Border Recon in 2011, a nongovernment organization which provides intelligence to the US Customs and Border Protection. Foley has set up about a dozen cameras in the Sasabe, Arizona, desert and shares a database of smuggling activity that he says he captures with the Border Patrol. "It's a huge game of hide and seek," Foley told CNN. "A wall might help a little bit. But we have a wall here, or a fence, or whatever you want to call it. And it's not a deterrent -- it just slows them down for 10 seconds. So unless you have people watching the wall, it's not going to do anything. You need boots on the ground." Hide Caption 16 of 20 Photos: Views from the border There are about 700 miles of fence along the 1,933-mile international US-Mexico boundary. This stretch in Sasabe, Arizona, has a fence that was built in 2007. "I think a barrier, a physical barrier, is definitely necessary," Shawn Moran, vice president of The National Border Patrol Council, told CNN. "Putting up a fence, putting up a wall, has stopped the vehicle loads from coming across the border. It has been almost 100% effective in doing that." Hide Caption 17 of 20 Photos: Views from the border The westernmost part of the U.S.-Mexico border overlooks the Pacific Ocean, inside California's Border Field State Park outside San Diego, and adjacent to a public beach in Tijuana, Mexico. This area is known as Friendship Park/El Parque de la Amistad, a historic meeting place where generations from both nations have gathered to visit with family and friends "across the line." Hide Caption 18 of 20 Photos: Views from the border The Galvez tunnel was discovered by US authorities in 2009, approximately 20 yards north of the national boundary in Otay Mesa, California. The tunnel is about 6 feet high, 4 feet wide, and goes about 70 feet below Earth's surface. According to Homeland Security Investigations special agent Juan Munoz, their San Diego-based task force has discovered more than 29 of these sophisticated tunnels in the past decade. Hide Caption 19 of 20 Photos: Views from the border A US Customs and Border Protection officer patrols the fence in Otay Mesa, California. Patched holes in the fence are a common sight along this section of the border next to Tijuana, Mexico. Hide Caption 20 of 20

The rate of drug seizures on its own can't be used as a definitive measure of success in stopping the flow of drugs, since it's impossible to gauge how many illicit substances still made it through the border and how many traffickers decided not to move their product in the first place out of fear of apprehension. But it's a widely used indicator of the trend lines in drug trafficking at various ports of entry.

Halfway through fiscal year 2017, US Customs and Border Protection has recorded 5,258 combined drug seizures along the Mexican border, on track for slightly less than the 11,459 documented in 2016.

"If we've created a barrier to drugs coming a certain way, they're going to find another way to do it," Lapan said. "So just because drugs are down in border patrol doesn't mean that we've gotten control or that it has stopped."

Some experts warn that the focus on building a wall without improving security at existing entry points will do nothing to stem the flow of drugs into the US overall.

"The wall will redirect the nature of smuggling but will prove to be spectacularly ineffective when it comes to actually stopping the flow of drugs," predicted Vanda Felbab-Brown, a senior fellow for global drug policy at the Brookings Institution.

Despite these criticisms, Secretary of Homeland Security John Kelly has supported the project. But he stopped short of Trump's claim to the Associated Press last month that, "We'll stop all of it. ... The wall will stop the drugs."

In congressional testimony in April, Kelly acknowledged that illegal drugs coming across the Mexican border "mostly come through the ports of entry" and other routes that would not directly be impacted by the construction of a blockade.

Air and water vs. land

For years, the Coast Guard, Customs and Border Patrol, Drug Enforcement Agency and other federal and local law enforcement agencies have confiscated large quantities of drugs headed to the US by air and water -- channels that would remain unimpeded by a wall.

Drug seizure statistics for nearly every agency involved in this coordinated effort have fluctuated in recent years, but they do indicate certain trends about how different drugs are moved in different ways.

The amount of cocaine seized via sea and air is significantly higher than the amount along land-based routes -- the Coast Guard intercepted 228,564 pounds of cocaine from October 2016 through March 2017 compared to 2,899 pounds of cocaine confiscated by border agents at land-based ports of entry along the southern US during that same time.

In 2016, the Coast Guard also seized dramatically more cocaine than its colleagues patrolling land-based entry points along the southern border, preventing over 400,000 pounds from entering the US compared to 4,000 pounds collected en route via land from Mexico.

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Additionally, the air and marine branch of the US Customs and Border Patrol reported that it seized 18,653 pounds of cocaine moving through routes in the Caribbean over that period.

While air and sea routes seem to be the preference for moving large quantities of cocaine to the US, agents at the southern border have confiscated dramatically more marijuana than did law enforcement in other regions.

The more than 500,000 pounds of marijuana seized by land-based border patrol agents through March of fiscal year 2017 dwarfs quantities collected by any other major law enforcement group.

However, Kelly told NBC last week that marijuana "is not a factor in the drug war," and instead placed higher importance on methamphetamines and heroin, as well as cocaine.

Law enforcement groups have seized more methamphetamine and heroin along the US-Mexico border than other smuggling routes -- but they've intercept significantly fewer shipments, and in total smaller quantities, of those drugs overall than marijuana and cocaine.

Halfway through fiscal year 2017, border patrol agents have intercepted more than 4,000 pounds of methamphetamines and nearly 500 pounds of heroin along the Mexican border -- amounts that are comparable to those recorded in 2016 and 2015 but pale in comparison to the haul of more than 500,000 pounds of marijuana and over 200,000 pounds of cocaine seized across all law enforcement departments.

Kelly highlighted Mexico as the primary source of foreign methamphetamines and heroin entering the US.

But a wall may do little to stop the flow of these substances, since Mexican drug cartels tend to hide their product in legal cargo and drive it through checkpoints at the border, Felbab-Brown said -- points of entry that will continue to exist no matter how high a wall might be.

And more than 350,000 vehicles, 135,000 pedestrians and 30,000 trucks move through 167 border crossings every day, according to the General Services Administration, which operates land-based ports of entry. At that volume, it is nearly impossible for agents to catch even a large portion of illegal shipments or to know how much drug cargo is getting through.

"Law enforcement is never a complete solution," according to Jim Plitt, a deputy special agent working for Homeland Security's investigative arm in San Diego and primarily focused on targeting the groups directing smuggling operations into the US.

A tall bill

Trump's plan to add thousands of additional border agents could help combat the flow of these substances through these checkpoints, but hiring could take five to 10 years and cost hundreds of millions of dollars.

Despite congressional approval of a bill that would add $ 1.5 billion for broader border security through September 2017, finding enough future funding for a wall in addition to 5,000 more border agents and immigration officers, increased detention capacity and more immigration judges remains a tall task.

DHS has limited ability to move money around in its budget, and would almost certainly be unable to fund most of the Trump administration's border enforcement plans on its own.

GOP leaders have already been put on notice by members of their own party that more spending had better come with a way to pay for it -- a point that likely means lawmakers will be forced to make significant cuts to other federal departments or choose between a wall and beefing up additional border measures.

Outside of broad cost estimates, few details are known about the wall save that the administration intends to pay for it with significant cuts in other areas.

Early versions of Trump's spending bill indicated the wall would partially be paid for by cuts to the Coast Guard -- the lead federal agency for maritime law enforcement -- but that proposal sparked criticism from lawmakers on both sides of the aisle and is not a part of the bill that negotiators have drawn up to keep the government open through September 2017.

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But neither is Trump's wall.

Bipartisan congressional negotiators opted not to allocate any money towards the project in the $1 trillion spending bill approved by Congress on Thursday.

Kelly, the secretary of Homeland Security, testified in February that DHS needs not only additional resources at these land-based checkpoints but also to intercept cargo along water routes and tunneling systems.

Whatever budget ends up passing, law enforcement will have to cope with it -- a reality that will endure whether or not a wall is built -- said Plitt. He noted that law enforcement groups are tasked with taking the resources they are given and using them to the maximum effect.

"While criminals wake up and say 'what do we want to try,' law enforcement wakes up every day and says 'what do we have,'" he told CNN. He called it a constant challenge to move resources to "where criminals are or where they are going to be."