My cousin has a cottage up north on a lake. Every summer a bunch of us guys and girls will have a get together for a weekend. Every night we go down to the lake and go skinny dipping. We have fun and the last time we were up there I wound up hooking up and having sex with a friend of a friend. She was fun to be around and one night when we were all skinny dipping everyone had got out of the lake to go back to the cottage and we had stayed. We didn't waste any time and started making out which led into sex. We were in waste high water and she jumped up onto to me and we started having sex as I was still standing in the water. We didn't go into any other positions other than that one because it was really the only position that was most comfortable. After she had come I lifted her off of me so I could pull out, started jerking off and wound up cumming in the lake. That turned out to be another great weekend at the cottage.
Though not very common in the urban and rural ares in this country (it is fairly common in tribal regions though), I've had my fair share of going skinny dipping right since college. it used to be with friends most time. Not sex always but yes there were times when for the thrill of it we've had sex while in the water. The underwater sex was most thrilling and I think it's the most awesome feeling to be in water open to sky (day or night - both have their own charm) <3 :D I won't call that bad though :)
Skinny dipping is NOT sex in a pool. It's doing NORMAL pool/beach activity w/o anything on, and done in a setting of multiple people doing the skinny dipping (and I say it only counts if people of both sexes are participating). Make-out sessions/sex in a pool or on the beach do happen, but are not part of the "group" activity known as skinny dipping. And for the uniformed out there, a girl can have sex in the pool while her swimsuit is still on.