One player is chosen as "It". This player closes his/her eyes and tries to find and tag the other players without the use of vision. The player who is "It" shouts "Marco" and the other players must respond by shouting "Polo", which "It" uses to try to acoustically locate them. If a player is tagged, then that player becomes "It".
The game can also be played on land, with slightly modified rules. It is similar to Blind man's buff where one person is blindfolded while others choose hiding places around the room.