Straightforward trope -- the hero is that much cooler because he not only has a Cool Boat, he actually lives on it (in theory, the hero could live on an uncool houseboat, except that houseboats are almost inherently cool), making it, at the same time, a Cool House. This may be symbolic of the hero's cool detachment from things, since he could (in theory at least) just raise anchor and move his whole life away whenever he wishes. Doesn't apply to people who live on a boat which serves as the home base for dozens or hundreds of occupants, or to those who just work on a boat owned by someone else, even if that's where they spend their nights (in a navy, as a pirate, on a cruise ship).