US swimmer Diana Nyad, 61, flashes the victory sign before her attempt to swim to Florida from Havana on Sunday. Desmond Boylan / Reuters

HAVANA - With a calm sea before her and destiny on her mind, 61-year-old US swimmer Diana Nyad plunged into the Straits of Florida at dusk on Sunday to begin what she hopes will be a world record 168-km swim from Cuba to Florida.

Wearing a black bathing suit and blue swimming cap, she methodically stroked her way north through crystal clear waters, accompanied by a small fleet of boats that included Cuban well-wishers seeing her off the island and the 45-member team that will accompany her on the 60-hour journey.

"I'm standing here in the prime of my life," she said before jumping into the sea. "Now I look out at a dead, flat calm, so I think this is my day."

Onlookers at the Marina Hemingway on the outskirts of Havana applauded and shouted "Buena suerte" - good luck - as she swam away with a fiery orange sunset to her west and encroaching darkness from the east.

Nyad waited until the daytime winds had died so she could start her quest in quiet seas.

Weather forecasters had predicted doldrum-like conditions until Thursday in the straits that separate the United States and Cuba, giving her a good window for the grueling voyage.

Nyad tried the crossing from Cuba in 1978 when she was 28, but failed in the face of winds and heavy waves.

The same swim was completed successfully by Australian Susan Maroney in May 1997. But Nyad's claim to a world record will be that unlike Maroney, she is doing it without a shark cage in the strait's warm, shark-infested waters.

Instead of a cage, Nyad will be protected by a surrounding electrical field and by divers who will watch for sharks and drive them away if they get too close.

Maroney was only 22, but Nyad said her comparatively advanced age is one of the reasons she will try the swim.

"I retired when I should have, when I was young, and a couple of years ago turning 60 I didn't want to feel old yet. I started thinking what if I went back, what if I went back to the elusive dream of Cuba," she said.

"I started training and found it was in my heart and in my body," she said. She said she hopes the swim will help people her age and older realize they still can accomplish many things.

"I want to be there to say we have many, many years of vitality and strength and service left in us," she said.

She also wants her effort to help US relations with Cuba.

Nyad was raised in south Florida and said she has had a lifelong fascination with Cuba and a longing for an end to longstanding US-Cuban hostilities.

"I hope my little swim is going to be a small symbol of the connection that we all know is coming very soon," said Nyad.

She retired from swimming and took up sports journalism more than 30 years ago, but still has the muscled body and broad shoulders of a marathon swimmer.

In her heyday, she set several world records, including swimming around Manhattan in 1975 in less than eight hours and a 165-km swim from Bimini to Florida in 1979.

None of those swims approached this one in length of time, and her longest training swim for the Florida Straits crossing was 24 hours.

She has five boats and two kayaks alongside her, which will keep her on course as she crosses the difficult Gulf Stream current, provide her food and water at regular intervals.

She said no sharks would be killed to protect her because it is important for the ocean ecology to preserve the predators. "We are not going across this ocean to destroy their territory. This is their world."

Reuters