An Australian man whose wife disappeared while they honeymooned at sea has been sentenced to eight years in prison for involuntary manslaughter.

Key points: Isabella Hellman went missing while sailing with her husband in the Bahamas in May 2017

Isabella Hellman went missing while sailing with her husband in the Bahamas in May 2017 Lewis Bennett was found on a life raft with $US100,000 in stolen silver coins

Lewis Bennett was found on a life raft with $US100,000 in stolen silver coins He was originally charged with murder but the charges were later dropped

Lewis Bennett, 42, apologised to the family of Isabella Hellman during a hearing before US District Judge Federico Moreno in Miami. He waived his right to appeal.

Defence lawyers sought a seven-year sentence, a year lower than the maximum number spelled out in a plea agreement reached last November.

Bennett, an Australian and United Kingdom dual citizen, was originally charged with murder, with investigators alleging he intentionally tried to sink the boat. The charges were later dropped.

Before Judge Moreno announced his decision, Bennett asked that he be let out of prison sooner so he could continue to raise the couple's daughter, who was an infant when her mother vanished.

"If you may permit me to be with my daughter as soon as possible," Bennett said. "I want to bring her up in a manner that is respectful to my wife's wishes."

But Judge Moreno sided with prosecutors and chose eight years in prison as the sentence and three years on supervised release.

Lewis Bennett pleaded guilty to the involuntary manslaughter of Isabella Hellman. ( AAP: Palm Beach County Jail )

Ms Hellman disappeared as the couple sailed off the Bahamas in May 2017.

A statement from the US Attorney's office said Bennett had sailing experience, including training on emergency procedures and had previously travelled from St Maarten to Australia.

His wife, a naturalised US citizen born in Cuba, had not trained in emergency sailing procedures and had less experience.

The couple had been married for three months when they set sail to St Maarten, Puerto Rico and Cuba for a delayed honeymoon in late April.

After they left Cuba on May 14, Bennett asked Ms Hellman to take over control of the boat for the night so he could rest in the boat's cabin, according to court documents.

He did not require her to wear a life jacket, harness or personal locator, prosecutors said in the statement.

He said he woke up when the craft hit something, and Ms Hellman was missing.

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Assistant US Attorney Kurt Lunkenheimer said Bennett "did not search for her diligently enough despite him being an experienced sailor".

The Government maintained he did not use the satellite phone to call for help. Instead, he loaded provisions and stolen silver coins onto a life raft and boarded it. Prosecutors said he called for help 45 minutes after he woke up and realised his wife was gone.

A US Coast Guard helicopter rescued him and flew him to the Florida Keys.

Authorities searched for Ms Hellman for four days, but never found her body.

"[Ms] Hellman's death occurred as a result of Bennett's knowledge of circumstances that existed that could have reasonably enabled him to foresee threat to life," the US Attorney's office statement said.

The FBI said an inspection found holes in the hull were inflicted from the inside and hatches were opened in a deliberate attempt to sink the boat.

Bennett had already pleaded guilty to transporting $US100,000 ($144,400) in stolen coins.

A lawyer for Ms Hellman's family said her relatives were satisfied with the judge's decision.

AP