A Water polo cap is a piece of headgear used in water polo and a number of underwater sports. The caps are used to identify both the player and their team, and to protect their ears from injury possibly caused by a water polo ball hitting the head.
Caps are differentiated by the number printed on them as well as the color. Players from the visiting team will wear light colored caps, whereas the home team players will wear blue caps. The goalkeepers wear quartered red caps, numbered "1", while substitute goalies have caps which are either numbered "1-A" in NCAA games, or "13" in FINA international games.
In underwater football, underwater hockey and underwater rugby, water polo caps are worn by competitors to identify which teams they are playing for, and to offer some protection to individuals against the possibility of a burst eardrum caused by the blade of a fin making direct contact across the ear. Opposing teams in underwater hockey and underwater rugby wear either white or dark (i.e. blue or black) caps while the water referees wear red caps. Underwater football appears to follow this precedent. Caps are also used in Aquathlon (underwater wrestling) for identification purposes; one competitor wears either a red or blue coloured cap while the other competitor wears a white or yellow coloured cap.