Something that exists or happens underwater exists or happens below the surface of the sea, a river, or a lake. giant submarines able to travel at high speeds underwater Some stretches of beach are completely underwater at high tide Underwater is also an adjective. underwater exploration. a retired underwater photographer
of, relating to, designed for, growing, living or used beneath the surface of the water
Underwater devices are specially made so that they can work in water. underwater camera equipment. a pool of clear water lit by underwater lights. below the surface of an area of water, or able to be used there