At least 3 people, but more people make for more fun
A player, designated It, closes his eyes for a set number of seconds while opposing players disperse in the pool. It, with eyes closed, then swims around the pool, attempting to tag the other players. It may detect the presence of others by feeling around with his limbs, or by sound, calling out “Marco!” The other players, regardless of their location, must respond, “Polo!” When the It player succeeds in tagging another player, the latter becomes It.
Swim, splash, and smile. Nothing too complicated here.
“Fish-out-of-water” allows non-It players to exit the pool to avoid being tagged. If, however, It should happen to catch that player out of the water by yelling “Fish out of water,” that player then becomes It.