You can buy a personal submarine for $2 million dollars. If you don't like the additional costs that maintaining a sub entails, though, renting will probably more your style. And that's exactly what U-Boat Worx is offering with their fleet of submersibles.
Two models are available: the C-Explorer 2 and the C-Quester 3. As the numbers in the name indicate, the former can hold two people (1 pilot, 1 passenger) and the latter can hold three (1 pilot, 2 passengers), with everyone sitting inside a single cabin with a see-through acrylic hull. Designed for maximum sightseeing, each one allows a 360-degree view while sitting inside the vehicle, so you can see water and marine life everywhere you turn.
Both watercrafts have a maximum operating depth of 330 feet, with the ability to stay under water for eight hours at a time (and up to 96 hours of emergency life support on board). A pilot and a surface officer with all necessary support equipment come with the rental, so you'll be taking supervised dives every time. Additional options you can request include a sampling arm (so you can take specimens of whatever you find in the water), HD video cameras (you never know when you can catch a glimpse of a mermaid down there), external lights and a bottom sampler.
U-Boat Worx will transport the subs directly from their place in Norway to anywhere in the world and can also prepare an itinerary for dives when you hire them for sightseeing. Price is steep, though, with rates starting at €75,000 for one month.