Before the watercraft was deleted, it had 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.
The Halo Encyclopedia lists a vehicle of the same name which it describes as a modified snowmobile, used for quick movements over snowy ground, particularly for medical transport, messaging, diversionary tactics, and evacuation. Its physical descriptions is otherwise essentially the same as the boat version, noting that the "nausea seat" proved to be more of a hindrance than a help. The Doozy is one of the three vehicles based on deleted material featured in the Encyclopedia, the other two being variants of the Warthog.