A neon green swimming cap glided through the water Thursday at the Mandeville harbor. Catching sight of it energized a group of onlookers, who had gathered on the shore to witness the final minutes of a herculean act.

“You can see his arms now!” a man walking his dog shouted, and a cheer erupted from the crowd.

“Go, Matt!” a woman exclaimed.

Seconds later, Matt Moseley, a 47-year-old long-distance swimmer, crawled onto the sandy dock on his hands and knees. With goggles on and sunscreen plastered across his face, he looked as bewildered as a hibernating bear suddenly thrust into the sun.

After a wobbly moment, the Lafayette native found his land legs and raised his arms in triumph. He had just completed a 25-mile, 15-hour swim across Lake Pontchartrain.

His friends mobbed him and popped open a bottle of Brut Champagne to celebrate.

Moseley, who lives in Boulder, Colorado, made the swim as a fundraiser for the Lake Pontchartrain Basin Foundation, which is celebrating its 25th anniversary.

It’s the first solo swim made across the lake, he said.

Moseley said he observed “English Channel rules,” meaning he stayed in the water the entire time, used no flotation devices and didn’t have physical contact with anyone else.

He did, however, have heaps of moral support from a crew of friends and relatives who made the journey alongside him in two party boats and a sea kayak.

“Swimming is not a solo sport; it’s a team sport,” Moseley said minutes after leaving the water. He then took a seat on a nearby boat, where he enjoyed an ice-cold Coke.

Moseley had originally planned to start his swim Tuesday night, but bad weather forced him to postpone it a day. He departed about 9 p.m. Wednesday from the New Canal Lighthouse in West End and finished about noon Thursday.

Moseley and his crew said weather conditions were less than ideal, with heavy swells making the journey more arduous than expected.

“I was getting tossed around like a sock puppet,” he said.

Frank Marcinek, who accompanied the voyage on a boat named the Bull Shark, said members of his crew got seasick from the rough waters.

“Waves were crashing all over the boat,” he said.

According to Quincy Castillo, who also rode on the Bull Shark, it was Moseley’s determination that inspired others along for the ride.

“That cat is superhuman,” Castillo said. “He stayed the course. It was amazing.”

Moseley said one of the most difficult parts of the swim was eating. He fed himself every 25 minutes, gobbling down everything from protein bars to eggs that were delivered to him with a feeding stick.

Moseley has been doing open-water swimming for years, in locales ranging from the Colorado River to the Florida Keys. Two years ago, he did a tandem swim across the lake with a friend and decided he would try a solo swim as a benefit for the foundation.

Moseley, who once worked at Commander’s Palace restaurant, said he wanted to raise awareness of how significant a resource Lake Pontchartrain is for the city. “It’s an important piece of the cultural city of New Orleans,” he said.

John Lopez, director of the foundation, said he feels the lake is still an “untapped resource” and that money raised from the swim will be added to other fundraising efforts to expand access and awareness of the lake.

Moseley, who currently works as a writer and communication consultant, has published one book, “Dear Dr. Thompson: Felony Murder, Hunter S. Thompson and the Last Gonzo Campaign,” which chronicles the work he did alongside author Thompson to free a Colorado woman they believed was wrongly imprisoned for murder.

In his swim across the lake, Moseley intertwined “gonzo” flair with a love for New Orleans traditions. He was accompanied by a boat with a giant mermaid on top, a crew of beer-swilling friends and a handful of musicians, including Papa Mali and composer David Amram.

Moseley said the music inspired him to keep pushing through the rough current.

Alfred “Uganda” Roberts, a percussionist who played congas on the boat, said there was a magical quality to jamming on the lake at dawn as Moseley pushed through the final few miles.

“We were so close to him,” Roberts said. “Every time his hand went for a stroke, I went for a beat.”