How is this "Why?", seems like standard Rule 34 to me.
Might be question why people would do this, like the Gardevoir and lopunny people.
Why, just why really is a good question. Like asking people why they would drown puppies or eat kittens. We know it happens and people do it, the question is still a simple...why?
It's not like any of this is new or original or even all that uncommon though. If we're applying it to any slightly non standard fetish the pool has no purpose