A water mask was a life support device that allowed a water-breather to survive outside of it's natural habitat. It consisted of a helmet filled with water and a backpack that contained a mechanism to assist in the life support process.
Later that year, and after being mutated by the Aquan surgo-op, James T. Kirk and Spock became water-breathers and had to be kept in an aquarium in sickbay. While en route to the surface of the ocean planet Argo to discover the source of their mutation, they wore an aquatic support apparatus so they could breathe. Similarly, the Aquans wore water masks while they visited the USS Enterprise. (TAS: "The Ambergris Element")