Scatter a variety of items on the bottom of the pool, such as dive rings (pool supply shop), spoons, coins, etc. -- anything that does not float. CAUTION: Do NOT use any items made of glass and that has sharp edges.
For team play, place two of every item (one for each team). Designate the shallow end of the pool for toddlers and little kids with the deep end for older children and adults.
Provide each team with a list of items to retrieve and pencils on a clip board for ease of writing.
First team that collects the most things on their list in the allotted time wins; the other team is then the 'salvage team' and has to scour the pool for the remaining stuff.