It's an awesome superpower to be sure, but on the other hand, they're supposed to be pirates. Why not kill the marine? (or let him die, since, yanno, the cannonball apparently would've taken his head off if Law hadn't done it first) Then again, OP definition of pirate is less "ruthless killer and thief" and more "adventurer", except when the pirate in question is a bad sort.

(I saw you post. Hope it'll pique someone's interest)