Pools do seem to be a natural setting for some tickling.
Years ago I saw a very pretty girl get absolutely wrecked by her boyfriend in a friend's pool at a party. They were in the shallow end, and he was holding her tight with one arm wrapped around her chest under her armpits and tickling her belly with the other hand. She was hysterical and kicking and splashing. She was clearly very ticklish. It lasted for a good couple of minutes too.
Our own pool has seen some ticklings too. I wrote posted a story a while back about my wife joining in tickling our hot neighbor which took place on the patio outside the pool and was preceded by the neighbor quickly tickling my wife inside the pool. Another time my wife was laying on a lounge chair face down and her friend was sitting on the chair next to her. When I came out to the patio I was asking my wife something and not getting a response. I asked if she was sleeping and her friend leaned over and tickled one of her soles. My wife jumped and shrieked and bent her knees up so that her soles were in the air but her friend continued tickling her. She stood up to get some more leverage and tickled both soles until my wife rolled over and sat up. It was pretty quick--maybe a minute long--but fun to see.