Before you close the question, take a look at the killer whale submarine :). I would buy one but I do not have $100,000. So is there a place in the world to rent one of those for one hour or so?
Clearly scuba can only get you so far, and while we've discussed submarine tours in the past, they are just jaunts. What's the deepest you can actually go as a tourist, and what about if you were ...