We All Live in a Yellow Submarine
US Submarines builds a variety of civilian subs for many purposes. Their most luxurious sub, priced at US $80 million, is the Phoenix 1000. Now that is a sub that almost anyone could live aboard. US Subs Seattle 1000 is smaller, costing only US$20 million.
The Integrity Subs model 68 also provides comfortable living conditions for a far lower cost, around US$ 5 million. Operational depth and range are more restricted than with the US Subs luxury submersible, however.
As the world sees more billionaires--particularly ones with a sense of adventure intact--you can expect to see them trying to outdo each other in adventure and luxury. This will take them places most people never go, such as the underseas, the high atmosphere, and into outer space.
Here is an interesting description of a US Submarines design for an "ambient pressure undersea habitat." Such habitats would not require special breathing gases nor decompression time. They would have to be designed differently than typical undersea habitats up until now, but the advantage of allowing untrained personnel to inhabit them would no doubt draw a large tourist crowd.
The undersea world contains many riches. Not nearly as many as the asteroid belt and the rest of the solar system, but then the undersea world is more accessible. It is likely that some of the billionaires who approach the submarine world as a novelty, may decide to invest there more seriously.