underwater

Beneath the surface of the water; of or pertaining to the region beneath the water surface.

“Watch out for underwater obstacles.”

“He was a pioneer in underwater exploration.”

nauticalBeneath the water line of a vessel.

“When the ship was brought into dry dock, it was found that she had underwater damage.”

figurativelyIn difficulty, especially financially.

financeHaving negative equity; owing more on an asset than its market value.

“We've been underwater on our mortgage ever since the housing crash.”

financeOf an option, having a strike price higher (call options) or lower (put options) than the current market price of the underlying asset or financial product; for example, an option to buy shares at $20 when the current market price is $15.

Growing or remaining under water

“viewing subaqueous fauna from a glass-bottomed boat”

“submerged leaves”

So as to go beneath the surface of the water.

“Can you swim underwater?”

Underlying water or body of water, for example in an aquifer or the deep ocean.

fishingA type of lure which lies beneath the water surface.

agriculturehorticultureTo water or irrigate insufficiently.

“Care must be taken not to underwater houseplants in the summer.”