Probably for operating an unregistered watercraft or not having a transponder. The kind of things maritime law requires for stuff like not getting shot be some idiot, not being IDed as a naval mine by the Navy, search & rescue, not hitting other ships, customs and border enforcement, etc, compelling border compliance, etc. There's a surprising amount of rules for doing stuff in the ocean.

Edit: apparently the vessel isn't technically seaworthy, there was a hurricane evacuation notice in the area, and he also threatened to kill himself and claimed to have a bomb. All of these things are usually guaranteed to piss off the government to varying degrees.