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

(fishing) A type of lure which lies beneath the water surface.

(agriculture, horticulture) To water or irrigate insufficiently.

(not comparable) Beneath the surface of the water; of or pertaining to the region beneath the water surface.

(nautical) Beneath the water line of a vessel.

(figurative) In difficulty, especially financially.

(finance) Having negative equity; owing more on an asset than its market value.

(finance) Of 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.

So as to go beneath the surface of the water.