Foam crafts are fun, especially with the pool theme. Make or embellish your own flip flops.
Noodle races (pool noodles), toss sinkable treasures on the bottom and send everyone down to fetch them. They must pick up one object, return it to their own bucket, and dive again. I put the strong swimmers buckets farther away from the majority of the prizes to give the less experienced swimmers a fighting chance. Once, I gave each girl a sand pail to decorate and said find a place to hide this for a game later. They had to leave the buckets in those places for the game...find a treasure, get out of pool, go behind the gazebo, and back to pool. It was kind of funny. I did encourage the less experienced swimmers to put it closer to the pool.
At a recent swim party, the dad held up hula hoops and the kids dove through them. I didn't think my daughter would do this as she was not diving yet, but she got in line, watched the other girls dive, and then when her turn came up, she slipped right through the hoop.
Have fun!
S