Super Soaker Free for All: Let each group shoot each other with the water guns until the water runs out then switch with another group.
Get some portable lawn sprinklers and just leave them on and let the kids run around through them.
Fire Hose: If you have a good relationship with your local Fire Department, they will be happy to swing by camp and flip on the water hose to soak the kids!
Water Ball Catch: Get a bunch of water balls (squish balls for the pool that hold water). Leave them out with a bin full of water and let the kids run around and play catch with them.
