Swimming and water aerobics, anyone?

Our community has a pool, and while it isn't really the best for doing laps in, it goes as deep as 6 feet and I've been doing basic water exercises for the past few weeks- different types of treading water, leg kicks, etc. I recently got a Hello Kitty beach ball and started doing "otter rolls," where you grip the ball to your chest and roll over with it repeatedly- it REALLY works your arms and core! I do two sets of 10 in each direction, but I'm building my strength to do more.

What are some of your favorite water workout routines?