I have an friend in the Navy, and our conversations center around ships and the Navy a lot. However, I keep making the mistake of referring to ships as boats. After several months, my friend finally straightened me out on this.
So when he introduced me to a submariner he knew, I asked, "What ship do you serve on?"
The response, directed to my friend, was, "BOATS! Subs are ----ing boats you surface-obsessed swabbie!"