You can play regular water polo... or you can play an ultra fun party version of water polo!
You'll need a pool (shouldn't be too much of a problem, if you're throwing a pool party:)), a rubber tube (the one you can use to float around the pool), two large goals and a few players... oh... and someone who wants to be the ball:)
So how do you play this special water polo game? One person is the ball. He or she sits in the rubber tube and floats around the pool. The other participants are players and are divided into two teams. Their goal is to push "the ball" into the opposite teams goal.
Since "the ball" is going to be pretty large (unless you use a small child), you'll also need large enough goals.
There are also some rules that should be enforced to make sure it's a fun experience for everyone (especially "the ball"):
Other than the it's up to you how you play. You can start from the center of the pool after each goal, or you can just give the ball to the other team. Where the teams start is also up to you (probably the best idea is to use the same rules as in regular water polo, each team starts on their side.
You can also play so that each team starts on their edge of the pool and they have to rush in the middle for the ball.
The details are up to you.
Have fun playing this funky version of water polo at your pool party!