An eco campaigner, who has raised thousands of pounds for charity, is thought to be the first person to swim around Grand Cayman non-stop.
Oly Rush, 37, completed the 95.5km (59.3 miles) swim around the Caribbean island in 36 hours and 59 minutes.
Mr Rush, from Upton, Dorset, said the "absolutely brutal" challenge was "the hardest thing" he has completed.
Last August, he completed a 15-hour swim around the Isle of Wight to highlight marine pollution.
The plasterer was able to walk from the sea unaided but was taken in an ambulance to hospital after completing his latest feat.
He was partnered by charities Plastic Free Cayman and Project Planet in his challenge, and has so far raised more than £18,000 online.
"I got round but it was absolutely brutal, it was so, so difficult," he said.
"It took me to some very dark places and I literally wanted to ocean to swallow me up."
"I was never going to quit because I was so motivated by the environmental cause and what humans are doing to the planet, but it was the hardest thing I've ever done.
"My food plan went out of the window after four hours and there were channels I had to swim through by myself because the support kayaks couldn't get through due to the swell.
"There were some sections where I was making little to no progress against the currents."
Mr Rush said there were "no major incidents" during the swim, as he battled through difficult conditions that left him with a tongue so swollen he found drinking difficult.
He added: "It is too early to say what I'll do next but I'll continue to raise awareness about sea pollution and no doubt when I get home I'll be on beaches clearing them of plastics."