Dolphins and Sharks
Start the game by gathering everyone into the middle of the pool. Divide half the players into dolphins and the other half into sharks, leaving one player to be the leader. Designate one end of the pool as the dolphins' safe point and the other end as the sharks' safe point. When ready, the leader calls out either "dolphins" or "sharks." Whichever group you called has to quickly swim away to their safety point while their opponents chase them, trying to tag as many as possible. Throughout the game, randomly call out either team. Any player tagged has to join the enemy team. The game ends when all players have become part of one team.
Colors
Pick one player to stand on the outside of the pool and play "It." The other players all gather at one end of the pool, each picking a color without having "It" hear them. "It" remains facing away from the players. When ready, "It" calls out a color. Anyone who chose that color has to quietly swim to the other end of the pool. If "It" hears them, he can dive into the pool and chase them in an attempt to tag them. Anyone tagged gets eliminated from the game. Play for several rounds until all but one player gets tagged and thus gets to play "It" in the next game.
Bumper Balls
Using a rope, section off a small area of the pool. Have all the players hop into that section, each holding onto a beach ball. On the count of three, the players all use their beach balls as a means to bump the other players into the rope, as if they're playing bumper cars. Anyone who touches the rope has to leave the pool. Keep playing until only one bumper remains in the pool, thereby winning the game.
Follow the Swimmer
This game works best with younger kids. Start by picking one player to be the leader and another player to be the watcher. All the players hop into the pool and line up with the leader in front. The leader then swims in whatever fashion she likes; anything from swimming under water to doing a back-stroke. All the other players have to mimic exactly what she does. The watcher, who stands looking from outside the pool, eliminates any players she sees who are not mimicking. Keep playing until only one player remains. He becomes the new leader.