The man was pulled from the vehicle about 2:15 p.m. and flown to a hospital in La Jolla, where he was pronounced dead, the Marine Corps at Camp Pendleton said. His name was being withheld until family notification.
Five other Marines escaped without injury from the training vehicle as it became submerged shortly before noon in the Del Mar boat basin in San Diego County's Oceanside Harbor, the Corps said.
Three instructors and three students from the Assault Amphibian School Battalion were conducting initial driver training when the vehicle sank, Maj. Daniel Thomas, the executive officer of the battalion, told the San Diego Union-Tribune. It wasn't immediately known whether the dead man was an instructor or a student.
They were operating about 300 feet offshore in water 30 to 40 feet deep while a companion vehicle waited nearby, he said.
"We are not placing a hold on training. But obviously we are going to take a look at this and then the commander will make a decision before anything goes back into place, to make sure any safety briefings that need to be given are conducted," Thomas told the Union-Tribune.
The Marine Corps said the incident was under investigation and it was not known what caused the vehicle to sink.
The 32-ton vehicle is a type of seafaring tank armed with a .50-caliber machine gun and can carry as many as 24 fully loaded combat Marines. - AP