To make our tiny swimming pool a bit more fun and interesting, on one of our 100+ degree days, I put a little bubble bath in while I filled up the pool. They loved it! When the bubbles start to disappear, I just sprayed the water again with the hose and they reappeared. Easy and fun!
Just for the record, regular diapers are really absorbent! I think I'll use a swim diaper next time, though. :)