Australian sailor Lewis Bennett has been charged with the second-degree honeymoon murder of his wife, with US authorities alleging he intentionally sank a catamaran during their private Cuba to Florida cruise.

The FBI’s surprise announcement came as Bennett, 40, was sentenced on Tuesday in a Miami court to seven months’ jail for transporting stolen gold and silver coins.

The coins were discovered on May 15 when a US Coast Guard helicopter rescued Bennett after he abandoned his sinking 12-metre catamaran he had been sailing with Colombian-born wife Isabella Hellman.

After months of speculation, the FBI used the coin charge sentencing to announce the second-degree murder charge.

Bennett, an Australian-UK citizen, had claimed he was asleep and he believed his wife was at the helm at around 1am when he heard a thud.

He told investigators his wife vanished and he abandoned the vessel into a life raft west of The Bahamas.

Bennet was also charged with stealing $140,000 in gold coins. Photo: PA

Ms Hellman was never found.

Authorities revealed a Coast Guard expert determined the catamaran had suffered intentional damage, not from a collision, but “from inside the vessel” in both hulls.

Two escape hatches were also open below the waterline, leading to flooding in the cabin, investigators alleged.

“The opening of both escape hatches is unexplainable as an accident and defies prudent seamanship,” a US Coast Guard Academy associate professor of Naval Architecture and Ocean Engineering told investigators.

“I cannot think of any items that would accidentally cause similar holes in both hulls at roughly the same time.”

Bennett allegedly used a satellite phone while in the life raft to call a colleague in Australia to give his coordinates and ask the colleague to notify the US Coast Guard.

Bennett took the couple’s one-year-old daughter Emelia, who was not on the catamaran, to England soon after the incident and she is being cared for by his family.

Ms Hellman’s family wants Emelia returned to the US.

A Coast Guard rescue swimmer who saved Bennett claimed Bennett’s backpack “was unusually heavy” when he was pulled from the lifeboat.

The $US100,000 ($140,000) in gold and silver coins were stolen from a sailing vessel Bennett worked on as a first mate in St Maarten a year earlier.

A search of Bennett’s Florida apartment allegedly discovered 162 gold coins hidden in a pair of boat shoes.

-AAP