An Australian woman will begin an attempt to become the first person to swim from Cuba to Florida without a shark cage tonight.

Chloe McCardel, 28, will battle strong currents, stinging jellyfish, sharks and her own physical limits as she attempts the 166-kilometre swim.

The treacherous body of water, known as the Florida Straits, is the Holy Grail for marathon swimmers and has been conquered only once, by fellow Australian Susie Maroney, who used a protective cage at age 22 during her 1997 swim that glided on ocean currents and enabled her to make the journey in just 25 hours.

"I will not wear a wetsuit. I will not use a shark cage. I will not hold onto anything. I will not get on the boat at any time," McCardel said ahead of the challenge.

But she will not be alone.

McCardel will be accompanied by a 32-person support team and will stop to eat and drink every half-hour.

Her ocean marathon should take about 60 hours - if she makes it all the way.

McCardel says it is the hardest swim in the world today.

"This is the penultimate of ultra-marathon swimming," she said.

"No-one has been able to do it and this is like the World Cup if you're into soccer, or this is like getting a world record and a gold medal at the Olympic Games, but it's possibly harder, possibly because no one has been able to do it."

McCardel's career includes swimming the English channel six times, with two double crossings.

Since 2011, four unsuccessful, uncaged swims have been attempted, three by American Diana Nyad and one by Australian Penny Palfrey.

In all, 20 people have tried and failed, McCardel told a press conference at the Hemingway Marina, west of Havana, where she will begin her journey.

ABC/Reuters