The Scubacraft is an open-cockpit submersible that's like a four wheeler, but under water. In order to use it you need to be in full scuba gear, but then it can take you down 100 feet with the push of a button.
The sweet pseudo-sub can also carry two additional passengers in addition to the pilot, if you've got some people you want to impress. The 250hp engine can keep you moving at a pretty good clip, and I assume it wouldn't be too tough to stick a harpoon gun on the front for shark deterring.
Sure, it's a whopping $120,000, but I'd like to see you tool around underwater this quickly with just a pair of flippers on.