Australian endurance athlete Chloe McCardel is stroking steadily through the balmy waters off Cuba in an attempt to become the first person to swim from Havana to Florida without a protective shark cage.

McCardel says she expects to spend about 60 hours in the sea before reaching the Florida Keys, a little more than 100 miles (160 kilometers) to the northeast.

On the way she'll be braving exposure, sharks and stinging jellyfish, and pushing the limits of human endurance.

She calls it "the hardest swim in the world today."

McCardel jumped into the water at Havana's Hemingway Marina on Wednesday morning. Accompanied by kayakers and a support boat, she began heading north.