well because when you have that much money it's really hard to spend it, and big boats are expensive. sports cars are relatively cheap and collecting them is for your star actors and low tier CEOs. islands are too expensive (i don't think they sell greek islands anymore) so only pedophiles that give "tax advice" can afford them. yachts are the right amount of expensive, between 100 mil to a couple of billions. at that range you can comfortably afford them, but they are not small enough to just be a drop in the bucket.

plus big boat cool. you can travel around the world and have the chance to pay filipino sailors fuck all for all their trouble. did you hear about the submarine garage? so awesome.

Conspicuous consumption? Differing boat cultures being common class signifiers (deep sea vs fresh water)? Also the sex pest stuff you said? The fact that the bourgeoisie are the only international class?

It's essentially making their own private mobile resort.

A mansion with everything you could ever want is "only" tens of millions of dollars, so any billionaire has as many as they could want. But you you want to go to different cities for different events, and settling in to a new place is such a hassle. You ever spend a week on vacation or at a relative's house and, for the first few days, every time you wake up in the middle of the night to take a piss, you have to remember where you are and where the bathroom is?

What if you were so rich that this is the biggest remaining problem in your life? Wouldn't it be nice if you just had one mansion set up the way you like, and could take it with you?

(Nobody should have this much fucking money.)