This is a great game for kids with goggles. The concept is simple: Choose a small, non-floating object and toss it into the pool. Have the kids race to locate the object and pick it up first.
There are lots of fun variations on the diving for treasure game. You can turn it into a longer challenge by tossing a dozen pennies into different parts of a large swimming pool and challenging kids to find them all. For stronger swimmers, you can play the game in the deep end.
When we were kids, we used pennies and rocks as the treasure, but now they sell special dive rings and dive sticks just for the game!