Doozy

The Doozy was a UNSC vehicle that didn't make it into Halo: Combat Evolved and Halo 2, and is seen in the Halo 2 Collector's Edition video. It ended up getting a chaingun in the back, and to man the chaingun, one had to sit in the "nausea" seat, which faced backwards.

Interestingly, in the Halo Editing Kit there is an option for vehicle types, and one is 'human boat.' This is because Bungie had built in a boat function into the game engine, but the concept was never used.