How about a game of piñata but instead of using candy - use water! Fill a medium-size garbage bag with 1-2 gallons of water (10 gallon bag works well). Play just like you were hitting a piñata. The winner is the child who manages to break the bag and unleash the "tidal wave".
(For youngsters that can swim the dog paddle)
Line up the kids on the edge of the pool. On go they jump in and all dog paddle to the other side, get out and bark 3 times. First one to finish wins.
How can I make an easy costume?