Wow! Water Moccasins? If they are in the pool, it's usually difficult for snakes to get out on their own, meaning there is usually a coping or a lip around the edge of the pool making it difficult for them to get out. Although I am no big fan of snakes, I would not kill them. Call a snake removal expert (since they are poisonous) I, personally would try using the net method with a long pole but don't want to suggest that to anyone only because of the danger of a bite from one of these guys. If you drain the pool, they'll still be in there and need to be removed. As far as "treating the pool to run off the snakes" as I mentioned before, difficult for them to get out. Give them an "escape route" Use a log, board, whatever... so they can get out. Add some chlorine to the pool (don't shock it, or it could kill them), but some chlorine is fine. It will irritate them enough to find that escape alternative that you've put into the pool.