Underwater means below the surface of the water. Underwater photography, swim underwater, an underwater cable.
Light behaves differently down there — red disappears within a few metres, which is why everything looks blue and why underwater photographers carry lights. Sound travels four times faster than in air. In finance, being underwater means owing more than a thing is worth. Note that the compound is under plus water.
1adjectiveunderwaterBelow the surface of the water.
“The divers explored an underwater cave full of colourful fish.”
underwater is a position below the surface.
Under plus water, a plain English compound.
From under + water.
Etymology from Wiktionary, CC BY-SA