McGrath's legal troubles might provide another clue in his Labor Day weekend disappearance, which led to a Coast Guard search that has since been called off without finding him.

According to legal docs, obtained by TMZ, McGrath pleaded guilty to aggravated assault in Tennessee in September 2022 -- his wife said he threatened to kill her and their kids -- and he ended up getting sentenced to 6 years of supervised probation.

The docs say McGrath got permission to move from Tennessee to Florida, where he was living with his parents ... and the cruise was a celebration for his father's 60th birthday.

But, McGrath's probation officers say he never got permission to go on the cruise, and it appears his probation office only found out he was aboard the ship when the media reported he'd vanished.

What's more, the docs say he's violating probation by failing to maintain communication with his probation officer. All attempts to contact him have been unsuccessful since the cruise, and his whereabouts remain unknown.

The PO also says McGrath is delinquent on $241.20 in probation fees, and he hasn't made any payments toward court fees. He's on probation for

McGrath vanished as the cruise was returning to Florida from the Bahamas. He was apparently last seen the night of Sept. 3 and his keycard was used to enter his room at 3:30 AM the next day. His family had planned to meet for breakfast, but he didn't show up and was reported missing when everyone got off the ship.

Cops say Carnival staff and security scoured the ship for McGrath ... and the ship's overboard detection system never alarmed. Bottom line: Police say there is no evidence he went overboard, and there was no sign of him onboard.

The U.S. Coast Guard brought in helicopters to search 3,300 square nautical miles for McGrath but didn't find a trace and called off the search.