During development of the original Halo: Combat Evolved, Bungie actually created a working boat. One map in particular was discussed during today's Bungie panel at PAX 2010 -- an area with several islands where the player would have to navigate from one to the other via watercraft. Bungie showed a video of an early rendering of the craft in use, which very much looked like a futuristic version of a cigarette boat.
In the end, it was cut from the game -- not because it didn't handle well, but because "it wasn't fun." Personally, we would've liked to try our hand at aquatic Haloing, but then again we're huge fans of Miami Vice.