An Australian marathon swimmer has started her record-breaking attempt to complete the 166-kilometre swim from Cuba to the United States without a shark cage.

Chloe McCardel, 28, hopes to arrive at Florida's Key West after 60 hours in the water where she will battle sharks, jellyfish, sea currents and her own physical limits.

"It's the hardest swim in the world today," she told reporters before making the plunge wearing a black bathing suit and rubber cap.

"I will not wear a wetsuit. I will not use a shark cage. I will not hold on to anything. I will not get on the boat at any time."

Under marathon swimming rules, McCardel cannot touch the boat or hang on to anything while she makes the crossing.

She will pause briefly every 30 minutes to gulp down a nutrient-fortified liquid meal from a bottle.

If she completes the swim as planned she should reach Key West at around midday on Saturday (AEST).

Only one person, fellow Australian Susie Maroney in 1997, has completed the Cuba-US swim, but she used a shark cage, which helps cut through the water.

McCardel is not using a cage but is instead surrounded by an electromagnetic field from a device held in the water which wards off sharks.

She is being accompanied by a 32-person support team which includes weather experts and doctors.

Her swim has been timed with the season and moon phase to minimise the presence of the venomous box jellyfish, which have plagued previous swimmers, including American Diana Nyad, who was stung repeatedly in August on her fourth failed attempt at the crossing.

Team of scientists to guide McCardel through Gulf Stream

McCardel will use a team of scientists in the United States to help guide her through the Gulf Stream, the powerful and unpredictable current that has stymied many previous attempts.

Chloe McCardel says she is doing the swim to raise money for cancer and improve Cuba-US relations. ( Reuters: Enrique De La Osa )

They will use real-time data on its currents to create computer models forecasting what lies ahead.

Last summer, British-born Australian Penny Palfrey got tantalisingly close to the Florida Keys but could not finish when she swam into the Gulf Stream and it pushed her in the wrong direction.

In theory, that kind of setback can be avoided because the crew of McCardel's accompanying boat, the Sunluver, will be warned and can lead her around adverse conditions.

Before setting off, McCardel and husband Paul McQueeney lathered her body to protect against the sun and chafing.

"I'm really excited, I'm trying to stay calm and relaxed and just think about the finish," said the blonde, muscled swimmer, whose past accomplishments include two back-and-forth crossings of the English Channel.

"It'll be tough, though. It's not going to be an easy ride, but we'll get through it as a team," she told reporters as she prepared to jump into the straits from a promontory at Havana's Hemingway Marina, the traditional starting point of Cuba-US swims.

McCardel says she is making the swim to encourage donations for cancer research, which can be made on her website www.chloemccardel.com, and to encourage better US-Cuba relations, which have been sour since the Caribbean island's 1959 revolution.

ABC/Reuters