The 57-year-old has been ordered to pay a fine of €600,000 (£502,000) and the court has also ruled that the boat be confiscated.
The Cagliari owner agreed to purchase 75% of the Championship club last month.
The takeover, which is yet to be ratified by the Football League, could now be in doubt following the guilty verdict.
The Italian businessman had been supporting the Elland Road outfit financially over the past two months, but he announced last week that he would not be injecting any more funds into the club until his purchase was approved.
Cellino's lawyer has confirmed that he plans to appeal against the fine.