Don, a Coast Guard investigation might not have anything to do with what happened underwater, in fact, unless the Captain ran them over, I don't think the Coast Guard would have any reason to investigate an underwater accident (though they do anyway for some reason).
If he was running spearfishing charters to federal waters without the proper permits, or had more than six paying customers on his boat, or had a dead goliath grouper on board, they'd probably be interested. Not saying that's what happened, just that they may be investigating him for something other than the circumstances of the accident.