Most people who spent any time around the pool as kids have played their share of Marco Polo or participated in relay races. There are plenty of games, though, that require little or nothing in the way of equipment but can nevertheless provide hours of enjoyment among family and friends.
Games are also a great way of coaxing young, apprehensive swimmers into the pool.
One very simple game is cork-scramble. This game is best played with a large number of contestants. When someone says go, everyone jumps into the pool and scrambles to collect as many objects as possible. When all the objects are collected, the player with the most objects wins that round. Of course, an underwater variation on this can be played using coins or other objects that sink to the bottom.