AUSTRALIAN sailor Lewis Bennett has entered a guilty plea to involuntary manslaughter in a Florida court, but just how his wife died on their private honeymoon cruise between Cuba and the US appears likely to forever remain a mystery.

Isabella Hellman went missing in the ocean off the Bahamas on May 15 last year as the couple sailed a 12-metre catamaran.

Hellman’s distraught family claimed she had met foul play at the hands of Bennett, a dual Australian-UK citizen, and police originally charged him with second-degree murder.

Bennett, 41, said his wife was at the helm at about 1am, he was awoken by a thump, searched the catamaran, could not find his wife and had to abandon the vessel when it started to sink.

When the catamaran was recovered a Coast Guard expert determined it 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.

Despite pleading guilty to the lesser charge, Bennett still claims he does not know what happened to his wife and mother of their two-year-old daughter Emelia.

Bennett is portraying Ms Hellman’s death as an accident caused by his negligence because he did not require her to wear a life vest or harness tethered to the catamaran.

The guilty plea ends the potentially sensational trial set to begin next month.

If the trial went ahead Bennett faced the prospect of life in prison if convicted of the murder charge while prosecutors had the potential stumbling block with a jury of being unable to confirm where her body was.

Bennett will be sentenced on January 10.

Prosecutors will recommend he serve eight years’ jail while his lawyers will not seek less than seven years.

“Although nothing can ever erase the pain and suffering caused by Lewis Bennett’s criminal acts, the US Attorney’s Office and our law enforcement partners hope that the defendant’s admission of guilt is a step toward justice for the victim, Ms Isabella Hellmann, and her family,” US Attorney for the Southern District of Florida, Fajardo Orshan, said.

“The federal government remains committed to the safety and security of our US citizens, whether they are at home in South Florida or travelling on the high seas.”

Bennett’s daughter is believed to be in the UK with his family.

Ms Hellman’s family in Florida are desperate to see her.

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

Prosecutors originally alleged Bennett committed the murder and sunk the catamaran near the Bahamas after Ms Hellman refused to follow his wish and move with their daughter to Australia.