Decorated bikini tops generally. And as for the point, it’s an entertainment profession, they’re kind of all pointless but the world would be dull without them.
It seems like a hardware-intense form of entertainment.
Since they mentioned swimming, I assume some sort of pool or tank is required, and likely transparent to view the performance. Not exactly an easy thing to use for other purposes between shows.
I’m just not feelin’ it, but I guess variety makes the world go ‘round.