Ablutofilia
This sexual arouse involves excitement from bath and showers. Especially very hot bath. This relatively harmless perversion is quite uknown in academic or proffessional literature.
I havent heard of ablutofilia until few years ago when one of the girl working in our club came with this story of her client. He always demanded that she has to rent a room only with a bath, not shower. Then she had to fill in with really hot water. He would lie down in the water, would be resting there for few minutes until he is excited enough (=erection comes) and then it was her time to step in to have sex with him. Which of course is a problem. The water is too hot for her. So they have to wait until the water is bearable for her but not too cold for him. Its the only way how he can achieve orgasm. Sometimes the water and overall uncomfortable situation is too much for her so they have to step out of the bath and continue having sex on bed. But for his climax they have to go back into the bath otherwise he would never finish.
To be honest I dont have any scientific explanation for this perversion. As I said academic literature on this topic is very subtile. On the other hand the ablutofilia doesnt have to be so rare as it seems. Having sex in hot sauna is actually very similiar. And sex in sauna is actually quite popular section on various porn websites. Unfortunately as I can stand being in sauna for maybe three minutes (I know I am horrible when it comes to sauna) this perversion will always be a mystery to me.