Origin: Germanic Old English prefix
Underwater has 6 different meanings across 1 category:
underlying water or body of water, for example in an aquifer or the deep ocean
"The ancient city lies underwater, hidden beneath thousands of feet of the Pacific Ocean."
In plain English: An underwater is not actually a noun because it describes something located beneath the surface of water, so you cannot define it as such without correcting that mistake first.
"The diver spent hours exploring an underwater cave system filled with colorful coral and fish."
to water or irrigate insufficiently
"The farmer decided to underwater his fields to save money on fuel, even though it meant the crops would struggle later in the season."
In plain English: To go down below the surface of water, usually by diving with special equipment.
"Please do not try to hold your breath while you are underwater watching the fish swim by."
growing or remaining under water
"viewing subaqueous fauna from a glass-bottomed boat"
"submerged leaves"
beneath the surface of the water, or of or pertaining to the region beneath the water surface
"The divers adjusted their gear before descending underwater to explore the sunken shipwreck."
In plain English: Underwater means being below the surface of water, so you can't see it from above unless you dive down or look through glass.
Usage: Use "underwater" as a standard adjective before nouns like "house" or "city," but avoid using it to modify verbs when describing the action of diving. Instead, use the adverbial form directly after a verb, such as in "the boat sank underwater."
going beneath the surface of the water
"The divers descended underwater to inspect the sunken shipwreck."
In plain English: Underwater means being beneath the surface of water, so you can't see it from above unless something is clear enough to look through.
"The fish swam underwater to find food on the ocean floor."
The word underwater is a straightforward combination of the prepositions under and water. It entered English simply by joining these two common words to describe anything located beneath the surface.