water column

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The vertical expanse of water lying between an area on the surface of a body of water and the floor directly below it but excluding the surface and the floor.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun hydrology A notional column of water from the surface to the bottom in a natural setting, notable for the differences in physical and chemical properties at various depths.
• noun The zone in which ascent and descent occur.