Updated 6pm

Divers searched off the Italian island of Sicily Monday for six people missing after a superyacht sank during a sudden storm, killing at least one person, officials said.

The 56-metre-long luxury yacht had been moored off Porticello, east of Palermo, when violent winds and rains swept suddenly up the coast before dawn, tearing through beach clubs and little fishing ports.

One of those missing is UK tech tycoon Mike Lynch. The yacht, Bayesian, was owned by a company controlled by Lynch's wife, The Times reported.

"It was terrible. The boat was hit by really strong wind and shortly after it went down," a British survivor called Charlotte told ANSA news agency.

The Bayesian, flying the British flag, had 22 people aboard, including 10 crew members, the coast guard said. The majority of those aboard were British, according to Italian media.

Survivor Charlotte said her young daughter had nearly drowned.

"For two seconds I lost her in the sea, then I grabbed her again. I held her tightly to me, while the sea raged. Lots of people were screaming," she was quoted as saying.

She managed to climb aboard a lifeboat, she said.

The yacht was anchored some 700 metres from the Porticello port when it was struck by a waterspout, which roughly resembles a mini tornado over a body of water.

"I saw the boat, it just had one mast, it was really big. I saw it suddenly sink," fisherman Pietro Asciutto told Ansa news agency.

'No longer there'

Another witness cited by Ansa said, "The boat was all lit up. Around 4:30 am, it was no longer there. A beautiful boat where there was a party. A normal joyous vacation day at sea turned into tragedy."

Another yacht in the vicinity provided help until rescuers arrived, media reports said.

Fisherman Fabio Cefalu said he and other fishermen rushed to the yacht's help too.

"But we didn't find anyone in the sea, we only found cushions and the remains of the boat," he told AFP.

Just a few hours later, the seas were calm as rescue divers recovered a body from the area the yacht went down.

Emergency services were spotted carrying a body bag to shore.

Survivors included two people with joint French and British nationalities, a Sri Lankan, a New Zealander and an Irish national, according to media reports.

Camper & Nicholsons, the managers of the yacht, said in a statement that they were "assisting with the ongoing search" for the missing people.

Italian authorities have opened a probe into the incident.

The Bayesian was a luxury superyacht built by the Italian shipbuilding firm Perini Navi in 2008.

It had a 75-metre mast and could reach speeds of up to 15.5 knots, according to the Boat International website.

A waterspout is a column that descends from a cloud to form a rotating mixture of wind and water over a body of water, often during severe thunderstorms, according to National Geographic.