The day after Hurricane Irma hit Florida, Randy Lathrop jumped on his bike at first light to assess the damage.

He soon spotted something exceedingly peculiar. The 62-year-old photographer from Cocoa, Fla., was pedaling along Indian River Drive under swirling clouds, he says, when he saw a rough wooden object resembling a canoe.

He took a photograph and texted it to an archaeologist friend, writing “WTF!!!!” His friend replied that it looked like an ancient dugout.

Was that even possible?

The mystery ships had arrived. Mr. Lathrop’s find, which set off an official effort to trace its provenance, was among a barrage of vessels and assorted flotsam half-sunk and washed up along southern Florida’s coast after the Category 4 hurricane in September.

An enigma washed up on Sept. 12 at Dania Beach in a state park near Miami, a 12-foot buoy with rust-colored vertical stripes. Park employees who dragged it off the beach noticed Russian writing on the side: “Гидрометслужба СССР.”

Randy Lathrop with the mystery vessel he found after Hurricane Irma. Photo: Randy Lathrop

No one on the spot could translate it fully, says park spokesman John Frosbutter. Eventually, local media published a translation, which The Wall Street Journal corroborated: “Hydrometrical Service of the USSR.”

Several federal agencies called about the enigmatic buoy, says Mr. Frosbutter, but no one from Russia called. The going theory, he says, is that it was blown 350 miles from Cuba, a Soviet stronghold during the Cold War.

The park set the buoy under a tarp without solving its secret, Mr. Frosbutter says. “I have no idea what we’re going to do with it.”

Monroe County, covering the 113 miles of the Florida Keys, had thousands of wrecked vessels after Irma, says county sheriff spokeswoman Becky Herrin. U.S. Coast Guard satellite imagery taken after Irma showed more than 2,570 distressed vessels throughout the state. The Coast Guard says 2,127 have been recovered or removed.

Among the guard’s strangest finds was on a boat named Cuki. On Sept. 19, a jogger happened upon the 45-foot sailboat resting on its port side on Melbourne Beach southeast of Orlando.

Aboard Cuki. Photo: Ian Gronosky

Some locals reported to police they thought they saw human figures aboard, says Matt Culver, the Brevard County Boating & Waterways coordinator responsible for post-Irma boat removal. “People thought there were bodies, like something weird happened.”

The Coast Guard boarded Cuki to check for survivors. In the cockpit were mannequins.

Photographer Ian Gronosky, who lives nearby in South Melbourne Beach, says he was one of the first to arrive at Cuki and saw mannequin parts everywhere. A torso and head lay under some life jackets, fishing nets and lines. A leg was nearby with a second near the rail.

“It was definitely creepy,” he says.

The boat’s hail port indicated on the stern was New Rochelle, N.Y., 1,109 nautical miles away—sparking speculation that it had floated that far.

Authorities from multiple agencies tracked down the 1974 fiberglass boat to a Floridian who bought it from a New York owner and kept it in Key West, according to vessel-registration records. The Coast Guard speculates it broke free and washed ashore 340 miles to the north.

The owner can’t collect his boat, Monroe County police say, because he is an inmate at Monroe County Detention Facility on Stock Island outside Key West.

Boats washed up along a Florida highway in September after Hurricane Irma. Photo: Joe Raedle/Getty Images

One lost treasure that reappeared post-Irma was the much-loved sign that has greeted visitors on U.S. 1: “Welcome to Key West, Paradise USA.”

The sign went missing after the storm. It turned out to be nearly 300 miles away in Fort Myers, Fla. A couple had found the sign blown down and took it.

A month later, the remorseful couple returned it, says Albert Gonzalez, president of the Rotary Club of Key West, which had donated the $8,000 sign. No charges were pressed, Key West police say.

David Ivler’s lawn. Photo: David Ivler

On Biscayne Bay, four sailboats ranging from a 35-foot cruiser to a 48-foot, 40-ton pirate-style ship wound up on David Ivler’s manicured lawn in Miami’s Coconut Grove neighborhood.

There they remain, leaking sewage, gas and rotten food, says Mr. Ivler, smelling “unbelievably putrid.”

Salvage operators are seeking up to $75,000, he says, to remove them. “It’s been an absolute nightmare.”

Questions still surround the vessel Mr. Lathrop found while on his bike. It had no motor, mast or sails. It was 15 feet long and weighed almost 700 pounds, he says. It had a pointed bow with a squared stern, and its nails were of square iron.

Mr. Lathrop suspected it was a canoe hollowed out of a cypress trunk using fire and tools. His research suggested the style dated to the 1700s.

“Look what Irma kicked up,” he wrote on Facebook. About 100,000 people shared the photos, some offering theories: It was a movie prop; it used to hang above the bar in a Trader Vics.

A post-Irma scene in Cudjoe Key, Fla., in September. Photo: carlo allegri/Reuters

Florida’s Division of Historical Resources retrieved the vessel and placed it in a water bath for preservation. The agency said it might have been long buried and dislodged from the ocean bottom during Hurricane Irma.

The DHR’s initial public verdict was that the canoe “is at maximum several hundred years old and minimum probably several decades old.”

Then carbon-dating tests came back, putting its construction in the 1600s. It was made from cedar, which is rare, says the DHR. It plans dendrochronology—testing the wood’s tree rings to determine age—for a more definitive date.

“This canoe is unique in that the radiocarbon dating indicates the wood is very old, but it has features that indicate it is more modern,” says DHR spokeswoman Sarah Revell. “So it is a bit of a mystery.”