Well, we tried in the bathtub too, but it was just too confining, so in a way it WAS painful. lol. But I think with the pool is that we didn't try to position ourselves near any jets. We were in the middle, not really by a wall, so maybe the fact that there wasn't any additional pressure besides the inherent water pressure made it work.
Or maybe it was the fact that she confided that she'd fantasized about doing it in a pool, but when the time came, she was nervous and I had to seduce her into doing it. Maybe THAT made a difference too. lol