To me the point is to have an activity WHILE ALREADY IN THE POOL. Most of you missed that part.
Sheets of stryofoam might make most MOCs float, or do a mostly submarine battle, where turn length is based on how long the player can hold his breath.
Have at least one player with 'depth charges' (water balloons or pool toys) that go off while the other player is trying to roll dice underwater. Or maybe just smack the water with an oar for a deafening underwater roar.
As for rules, the Steve Jackson rules for the Pirate Game are fun and free, and include looking for treasure, scuttling ships, etc.
have fun, stay cool, avoid the filter!