The estate of Paul Allen owns some of the world's largest yachts, and besides being luxurious yachts, they're also some of the most advanced research ships in the world. He lent one of them to researchers who found a battleship lost in World War II, and researchers aboard another one found a lost WWII carrier.
His estate also owns over 30 military aircraft, which are on display near where I live. (I haven't been there yet.) I'm not sure whether they're in flying condition, but I would assume that they have had their weapons removed or disabled.
https://beam.land/yachts/the-full-story-behind-paul-allen-s-billion-dollar-toy-fleet-1560
So yachts aren't necessarily just about luxury. They might be for science or some other purpose related to the owner's interests.
I have previously proposed the idea of a campaign where the patron (or a player character, if players are OK with status imbalances) is a dilettante mercenary, where profits from military action are desirable, but secondary to the goal of bringing glory to the dilettante. A militarized yacht would be a reasonable ship for such a campaign.