An underwater breather is a device used to allow a person to breathe underwater. It is also known as a GILL, which is a four-letter word that is one of the solutions to the crossword. A GILL is an organ found in fish and other aquatic animals that allows them to extract oxygen from the water. Underwater breathers use a similar process to extract oxygen from the water and provide it to the user. They are typically used by divers and other underwater explorers.