When Charlestown resident Scott Keeley went to the beach on a sunny Sunday recently, he thought he had state law on his side: the Rhode Island Constitution says he’s allowed to collect seaweed along the shore, and, he believes, nothing in it prevents him from sitting down to shake out the sand from his haul.

But a private security guard hired by nearby beachfront property owners had little interest in his arguments about rights under Article 1, Section 17, Keeley said. The paper copy Keeley had printed out didn’t work, either.

And when the South Kingstown police officer showed up, things really went “pear-shaped,” Keeley said. The officer, too, told him that the area parallel to the stretch of private homes on Charlestown Beach Road was private property and that he had to move along, he said.

Keeley disagreed, but he didn't want to get arrested, so as he prepared to leave and bent down to pick up the bag of seaweed, he saw another “really nice chunk” on the ground and picked that up too.

A video with tens of thousands of views on Facebook shows what happened next: The officer arrested Keeley for trespassing. Keeley really did want that seaweed, but he also was there to make a point and launch a protest.

“The only way this is going to change is through civil disobedience right now,” Keeley said.

Keeley’s arrest is an unusually vivid example of a debate that’s long burbled just below surface in Rhode Island, periodically crashing ashore and into public view: Where do the public’s rights begin and private landowners’ rights end along the water in the Ocean State?

Where it ended for Keeley was in custody for about a half hour. On a positive note, he commended the South Kingstown officer for his professionalism. But he has no idea what happened to the bag of seaweed he was going to use to fertilize his garden.

But this, Keeley says, will not be the end of it: He is considering challenging not just his arrest but the way that coastal rights are enforced in Rhode Island at a time when beach erosion, climate change and the increasingly aggressive tactics of private landowners, like hiring private security guards, are chipping away at Rhode Islanders’ rights to access the beach.

For now, Keeley won’t have a chance to fight it out in court, at least in this venue: The police chief in South Kingstown, Joseph Geaber, said Tuesday in response to a request for comment from The Providence Journal that the department would not be pursuing the charge against Keeley, noting the challenges in figuring out where the property boundary lies.

“It would be very difficult to prove the case beyond a reasonable doubt when it cannot be determined how to accurately determine the line itself,” Geaber said. “We apologize to Mr. Keeley but feel the officer was acting within the scope of his duties.”

At first, it seems like a straightforward issue: “The people shall continue to enjoy and freely exercise all of the rights of fishery, and the privileges of the shore,” Article I, Section 17 reads, with usages “including but not limited to fishing from the shore, the gathering of seaweed, leaving the shore to swim and passage along the shore.”

But how those rights are enforced plays out in a way that, like the clumps of ocean plant life Keeley was collecting, can be tangled, stringy and difficult to swallow.

Where is the boundary between public and private, and how do you calculate it? If people are on the right side of that line, can they sit in a chair? Can they stand there for an hour? If they can’t, how is one supposed to fish, or set down a towel after a swim? If that line is 15 feet out to sea in some areas, as the homeowners near the Charlestown Town Beach assert, how are people supposed to pass along and collect seaweed?

A state Supreme Court case, the State vs. Ibbison case of 1982, set the boundary for the purposes of the public’s constitutional privilege of shore access as the “mean high tide line.”

With beaches shrinking or growing with the forces of tides, erosion and rising sea levels, that’s a difficult concept to pin down.

Some people assume that the mean high tide line is wherever the farthest landward seaweed is. After all, it got there somehow. But measuring the mean high water line involves much more than that. It can’t be done with the naked eye, or in a day. The mean high tide line mentioned in the Ibbison case is measured in years, not days: You’d need 18.6 years and specialized equipment to separate the actual tide from the waves’ farthest reach to see where the highest place the tide — not the water — reaches on average.

“It’s something that’s really not visible at all,” said Janet Freedman, coastal geologist at the Coastal Resources Management Council. “You can’t put a permanent line, unless you want to be surveying every day.”

According to Keeley, the police told him that the private landowners at the beach where he’d been arrested have surveyed the property and determined that the mean high tide line is actually several feet out in the ocean water at all times, meaning the entire beach, and on into the water, belongs to the private homeowners.

The latest series of tide observations from the National Oceanic and Atmospheric Administration was collected from 1983 to 2001. But the beach has changed since then — in some areas, it's shrunk significantly.

Nobody in the area where Keeley was arrested has worked with the Coastal Resources Management Council, part of the state government, to get a scientifically rigorous measurement of where the mean high tide line is with recent data, Freedman said.

The homeowners who’d hired the security guard either didn’t respond to requests for comment or, in one case, declined to comment in a hotly contested dispute with neighbors.

The state has really good data about the ebb and flow of water in the area of Charlestown Beach, Freedman said, but that’s not the data the state uses to determine where the public’s right begins and private landowners’ rights end.

“In my opinion,” Freedman added, the mean high tide line “is not an appropriate way to determine where the water flows on the beach.”

Freedman’s recommendation for people who want to access the coast?

“Download a copy of Article 1, Section 17 of the state constitution,” Freedman said. “Laminate it, and bring it with you. Not that it made any difference for the guy who got arrested.”

The video of Keeley’s arrest shows just this sort of confusion over the mean high tide line. His friend, who shot the video, said he was on the right side of that mark, as evidenced by the seaweed on the beach that he was collecting. In fact, the waves were lapping at their feet, said fellow Charlestown resident Matthew Glander.

The homeowners, though, say the mean high tide line is several feet out into the water. If they’re right, how is anyone supposed to do the things the state Constitution gives them the right to do?

“Are you supposed to swim there with a fishing pole and cast your line from 15 feet out in the water?” Glander asked.

Read Porter, senior staff attorney with the Maritime Affairs Institute at the Roger Williams University School of Law and the Rhode Island Sea Grant Legal Program, said in an email that the homeowners' apparent assertion that the property line is always out in the water can't be true "as a matter of law."

"Because the property line moves with natural processes, beaches always include some public lands at low tide," Porter said. "Mr. Keeley and all other citizens of our state therefore should feel confident in exercising their constitutional rights to enjoy the rights and privileges of the shore — but it's a good idea to check the tide tables first."

James Bedell, a Narragansett resident and longtime advocate for the constitutional rights to coastal access, said Tuesday that the issue is so difficult it essentially means there is no beach boundary where people can’t go to collect seaweed, pass through or take a swim.

Bedell’s argument is that the state Supreme Court purposefully set the boundary at something unknowable to guarantee access to the coast.

“This is an impossible situation to tease apart,” Bedell said. “But remember, they did write that in the Constitution. This was a precious gift for Rhode Islanders.”

Whether the rules should give people the right to stop and set up a chair, or merely pass through, is a question for another day, Bedell said. Right now, private property owners are encroaching on those rights, Bedell said, hassling him sometimes even when he’s just walking through.

“There is no boundary,” Bedell argued. “The Constitution guarantees the right to do things. It doesn’t define where you do them.”

On a tour Monday of the area where he had his run-in with a private security guard and then a lawman, Keeley talked compromise: A few years ago a proposal would have meant a 10-foot buffer up from the water for public access, instead of an inscrutable one that might be submerged.

Better yet, Keeley said: “I’d like to go for every grain of sand” for public access.

Though he was sitting when he was first encountered by the security guard, Keeley noted that nothing in the state Constitution says that you cannot sit while carrying out your constitutional privileges.

On the morning of July 6, Keeley is planning a demonstration on that same strip of land, inviting people to walk, swim, fish, gather seaweed, and calmly, civilly protest. It is something of a line in the sand, moving farther inland than before.

“How can you possibly perform the actions in our Constitution with the way they’ve interpreted this law?” Keeley said before picking up some more seaweed. “You only have these rights if you’re a scuba diver.”