also: みずそこ
noun
water bottom; bottom of a body of water
Refers to the bottom of any body of water: sea, river, lake, pond, etc. Often used in descriptive or scientific contexts.
湖の水底には沈んだ船がある。
There is a sunken ship at the bottom of the lake.
水底の砂を採取して分析した。
We collected and analyzed sand from the water bottom.