This is a conversation I recently had with my buddy who wanted to show me around his naval base.
Me: "OK, so here we are on this ship--"
Him ..."This is a submarine. Submarines are 'boats'. Everything else in the Navy is a ship, but submarines are boats."
Me: "Oh. Er ... what about a gunboat?"
Him: "That's a ship."
Me: "Well, what's a gunSHIP, then?"
Him: "That's a helicopter."