Pool Battleship.

You know that game, Battleship. You know, it's kind of a guessing game with the "You sunk my battleship" and all?

Well, we modified it by all jumping into the pool and wrestling. If you hold somebody underwater until they give up (And then about 15 more seconds), that person is disqualified.
I usually lost, but it was a lot of fun, you know, overpowering people and holding them underwater.

I don't recommend that you try this without adult supervision. I mean, we did. But we were tough jackals. I imagine that most children of our generation and the next are kinda soft and easily killed.