Pop Culture

In his case, he does seem to just genuinely like the ocean and boats, and while his age and even more so health probably don't accommodate scuba diving he might be able to handle submersible trips. He has a history of funding marine ecology stuff and research submersibles and tagging along on the boats for those, so I assume it's literally that he's planning to live on the yacht at least part of the time, making it a half-billion dollar mansion with all his favorite toys on board.

No one should have that kind of money to throw around like that and the whole thing is a materially ludicrous waste of resources and labor just to cater to a rich guy just because he's retained a childlike ability to actually enjoy things despite his wealth, but it does seem to be a case of him just earnestly liking sea stuff more than the typical status symbol conspicuous consumption thing most of them are doing.

A yacht is basically a floating palace with sea moat. The submarine is the escape tunnel.