Surprised nobody has yet to mention US Submarines[1] their Phoenix 1000 design [2] would be (if anyone ever bought one) the largest personally owned undersea craft, and a helluva an RV. There was a BBC documentary on a guy who built a nominal atmosphere submarine that could go down 1000' (300 meters).
[1] http://www.ussubmarines.com/submarines/luxury.php3 [2] http://www.ussubmarines.com/submarines/phoenix_1000.php3