A young Frenchman suffered head injuries early on Saturday when he hit the boat he was jumping from at St George's Bay.
The police said the accident happened at about 6am.
Onlookers dived into the sea and pulled the 24-year-old to shore.
He was taken to hospital, where his condition was described as serious.
The police are investigating.
Magistrate Yana Micallef Stafrace is holding an inquiry.