Alcatraz is now a fully accessible tourist destination

Keith Rothwell from Caernarfon and Michael Pepper from Felinheli will swim the two-mile strait between the island and San Francisco, USA, for charity.

They are joining 500 people for the Alcatraz Sharkfest Swim on 16 July.

Despite the freezing waters, neither of the men is planning to wear a wetsuit for the attempt, unlike other swimmers.

They are aiming to raise money for cystic fibrosis charities, a cause close to Mr Rothwell's heart, as his nephew has the condition and recently underwent a heart and lung transplant.

The island, dubbed "the Rock", held high-security prisoners in its Alcatraz jail between 1934 and 1963, and although five men did escape from the prison without being recaptured, it was never established if they survived the journey to the mainland.

Alaska

Mr Rothwell said of the swim: "It's a couple of miles - it's not a huge distance, but there are two choices of ways of doing it.

"You can either wear a wetsuit or not, and we have decided to do it without a wetsuit.

"It's very cold because apparently the waters come down from Alaska, so it just never gets warm, although it's California."

The men plan to acclimatise to the cold by practising their swim in the waters at Dinas Dinlle near Caernarfon.

Mr Rockwell, 56, a chef in Beaumaris, told BBC Radio Wales he took up marathon swimming at the age of 39 in order to keep fit.

Since then, he has taken part in a number of races.

"It's called master swimming, for people over the age of 25, and you race against within your own five-year age group," he said.

"In 2003, we decided to do a relay swim across the English Channel, and five of us managed to do it.