When helping a teen plan party games, you have to walk the fine line between having fun and not being too childish. Teen water games provide everyone with the chance to let loose and enjoy themselves and their friends without sacrificing too much dignity. To make sure that every teen is involved in the water party games, emphasize teamwork and group victory rather than individual achievements.
Leaky Cup Relay
Divide the teens into two groups and give each one a large Styrofoam cup with a hole punched in the bottom (use a nail). Before the party, set up an obstacle course with tricky but not too difficult obstacles, such as circling a chair twice and jumping over a flower pot. There should be empty buckets at the end of the obstacle course. The first player for each team fills the cup with water and holds it over his head. Then, he runs the obstacle course and empties whatever water is left into the empty bucket. Teams continue until one team has filled the bucket to the designated spot.
Prepare the area much like a soccer field, with two goals and enough space to run. Arm each teen with a super-soaker-style gun. Any type will work as long as it sends out plenty of pressurized water. Instead of a soccer ball, provide an inflated beach ball. Let the teens play using traditional soccer rules, but instead of kicking the ball, the teens must use only the water from the squirt guns. Even the goalies must use a water gun to divert any shots toward the goal.
Divide teens into groups of four, then supply each team with two towels and a water balloon. Each pair of teens must hold the towel between them and toss the water balloon back and forth using only the towel. Teams must work together to launch the balloon from the towel, as well as move around to catch the balloon in the towel. The team with their water balloon intact the longest, wins.
If the teen water party takes place in a pool, get everyone together to play Octopus. One person is the octopus and starts at one end of the pool. Everyone else starts at the other end and attempts to reach the Octopus' side of the pool. The Octopus must tag people before the reach the other end. When someone is tagged, they become an Octopus' "arm." They must stand in place and can only tag someone who comes within arm's reach. Eventually the Octopus and all his "arms" will get everyone and the game starts over.