An ultramarathon athlete has survived a history-making swim in a river teeming with crocodiles, but he says the only Crocs he saw were the ones on his coach's feet.

Andy Donaldson has claimed a new world record after completing the 55km swim from Lake Argyle to Kununurra's Diversion Dam on the Upper Ord River, in Western Australia's remote Kimberley region.

The 35-year-old on Tuesday became the second person to pull off the epic effort, smashing the world mark set by Simone Blaser who completed the journey in 16 hours and 13 minutes in 2024.

In his swim, Mr Donaldson battled severe heat and dehydration to complete it in just 11 hours and 51 minutes, pending ratification by the World Open Water Swimming Association.

Speaking to AAP after sleeping "like a dream", Mr Donaldson said he was in high spirits, despite his body feeling the effects of the marathon effort.

"The bonus is I still have all my limbs," he joked.

The upper Ord River is inhabited by large numbers of freshwater crocodiles, which locals are accustomed to swimming with and are considered mostly harmless to humans.

"I didn't see a single one, other than the Crocs that my coach was wearing on his feet," he said.

"I don't know whether to feel relieved or disappointed."

But Mr Donaldson faced more serious challenges, with the last 10 kilometres pushing his body and mind to the limit.

"I felt like I was carrying an elephant on my back," he said.

"Mentally, I was fatigued, physically I was fatigued. I'd been sweating for over 10 hours, and I think I was quite dehydrated, so my muscles were starting to cramp really hard.

"The final five kilometres felt like it was never ending."

Mr Donaldson, who grew up in Scotland before relocating to Australia in 2013 at the age of 22, pursued Olympic swimming earlier in his career.

When those dreams didn't pan out, he switched to open water competition with immediate success, winning the Rottnest Channel swim in 2021.

He has a swag of world records to his name, including the 45km Manhattan Island circumnavigation, the 170km Bahrain Swim Challenge and the 104km Ibiza Swim Challenge.

Mr Donaldson said one the biggest dangers in open-water swimming was hypothermia, but heat and dehydration in the 28C waters of the Ord River were bigger issues.

"There's a bit of a myth that you don't sweat when you swim, but you certainly do," he said.

"I probably dropped a few kilos from finishing that swim, mostly in liquids.

"The mind always gives up before the body does ... I was in a lot of pain, but I was still well within my limits and could have kept going if I needed to."

He said he had not yet decided on his next challenge, and was looking forward to a well-earned rest.

"It's important to celebrate the achievement, lift up the people that have helped make it possible, and just reflect on the learnings from it before moving on," he said.

Previous swims have raised funds for mental health charities, including the Black Dog Institute and the Kai-Fella foundation, for which he is an ambassador.