Adjective

1. beneath the surface of the water, or of or pertaining to the region beneath the water surface
2. beneath the water line of a vessel
3. having negative equity; owing more on an asset than its market value

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

Synonyms

• (beneath the water surface): subaqueous
• (having negative equity): upside down

Noun

1. underlying water or body of water, for example in an aquifer or the deep ocean
2. A type of lure which lies beneath the water surface.
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