Bennett also didn't discharge flares during the hours he lay waiting for the Coast Guard to rescue him, prosecutors said.
In November, Bennett pleaded guilty to unrelated charges of transporting $5,000 in gold and silver coins, according to the complaint.
On Monday, Bennett, 41, pleaded guilty to involuntary manslaughter in the death of Hellman, a Cuban-born US citizen.
Hellman died as a result of "Bennett's knowledge of circumstances that existed that could have reasonably enabled him to foresee the threat of life to which his acts or failures to act might subject another, namely Ms. Hellman; and his gross negligence, amounting to wanton and reckless disregard for human life", according to the DOJ.
Bennett's admission says on May 14, 2017, he and Hellman, while on the last leg of a two-week sail around the Caribbean, had dinner around 8 p.m.
Federal officials said Bennett will now be held accountable for his wife's death.
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 said Bennett is an experienced sailor who received a certification from the Royal Yachting Association in the United Kingdom as a "Coastal Skipper".
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's attorney, Edward Downey, declined to comment on Monday.
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. Bennett figured he abandoned ship 45 minutes after being awakened.
The Coast Guard searched for Hellman for three days before suspending the search.
Bennett told investigators he gathered his personal belongings when he noticed that the vessel was taking on water, according to a criminal complaint filed when he was arrested. An FBI agent wrote in an affidavit then he believed Bennett intentionally scuttled the boat.
The FBI says an inspection of the catamaran showed portholes below the waterline had been opened and damage to the twin hulls appeared to have been caused from the inside. "I can not think of any items that would accidentally cause similar holes in both hulls at roughly the same time", according to the affidavit.
In court documents, investigators pointed to a predictable motive: the money and property Hellman brought to the three-month marriage that would revert to Bennett, backed by Bennett filing for a presumptive death certificate just four months after her disappearance.
Bennett is now serving a seven-month jail sentence after pleading guilty to transporting the coins.
"Although they already knew, they now have confirmation, that their beloved daughter and sister, Isabella, was never coming back to us, we now have confirmation that she was taken from us by her husband, Lewis Bennett".
He admitted transporting the coins and is now serving a seven-month jail sentence.