I do this in the middle of a work out or at the end. It's fun and the particpants love it.
Have everyone gather on one side of the pool. Have them squeeze into a tight group. Then everyone starts jogging across the pool quickly. During the jog I will start yelling "reverse" at which point they turn around and go in the other direction. I do this over and over again. We might jog all the way across and halfway back and I will yell reverse. Maybe we will only take 3 steps and I yell reverse again. This is great resistance work. I keep this up for 5 to 7 minutes.