Alex Ciardi, 47, an American married to Giuseppe Ciardi, an Italian businessman - the couple lived in London - had dived from the deck of the family's yacht Allelujah in the harbour of Cala di Volpe on Wednesday. She was intending to swim the few hundred yards to another yacht to join a party of friends for lunch.
A few minutes earlier an eight-metre motorboat owned by a Milan-based entrepreneur, Giovanni Mahler, had set off from the jetty of Cala di Volpe with a party of passengers. He was bound for his own yacht, Tanit. Among those waiting for him on the yacht were Flavio Briatore, a former close collaborator of the clothing billionaire Luciano Benetton and now managing director of the Renault Formula One Team UK.
The motorboat passengers maintain that their vessel was not going fast, and in fact could not have been going fast because the harbour is so congested. No one on board saw anything in its path. "We weren't aware of anything, " said Miki Gioia, a screenwriter, one of Mr Mahler's passengers. " We didn't see anybody swimming. The sea was slightly ruffled, which made it impossible to see the lady."
Five minutes after casting off, Mr Mahler heard a muffled thud. He turned to the only professional seaman on board, a man of Turkish origin who has not been named, and said: "We've hit something." Looking into the water they saw a spreading cloud of blood. Then they saw the body of Mrs Ciardi.
Her huband, a wine connoisseur and major investor in the Signature group which recently sold The Ivy restaurant in London, watched helplessly as his wife was struck by the motorboat's propellers.
He and his three children, jumped into Allelujah's inflatable tender and motored to the spot. Mrs Ciardi was still breathing and they succeeded in hauling her on board. They took her shore but by the time an ambulance arrived she was dead.
Mrs Ciardi, who worked for GLG Partners LP in the West End of London, had been married to Giuseppe, an investment banker, for 22 years. She also worked with the children's charity, the NSPCC.
Its director and chief executive, Mary Marsh, said: "Alex Ciardi was one of the NSPCC's most valued friends, a dedicated supporter who helped gain the commitment of others. She made an extraordinary contribution towards helping end cruelty to children. We are devastated."
In a statement, her family said: "The family is immensely saddened and grief-stricken...
"She brought sunshine into our lives and will live in our hearts for ever."
The accident occurred close to Silvio Berlusconi's holiday villa, and near Porto Cervo, where a year ago Mr Berlusconi went walkabout with the Blairs.