Line up everyone on the side of the pool and toss in "treasure." I have used toy animals (plastic), plastic or rubber rings and even small (1-2 inch) smooth rocks. See who can bring back the most.
An activity that children can never get enough of is the "car wash". Simply supply buckets of mild dishwater, sponges, brushes and rags and the children's riding toys. Sit back and let them at it. It's a good idea to have the kids in their swimsuits. Getting wet seems to be the big attraction here. Supply them with other outdoor toys that may need a "bath" too. Make sure they have their sunscreen on.
Like duck, duck, goose. But instead of touching the goose on the head, they wring out a wet sponge over their head.
How can I make an easy costume?