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.
Compound of 水 (water) and 底 (bottom). The reading すいてい uses the on'yomi of both kanji, while みずそこ uses the kun'yomi. The on'yomi reading is more common in modern Japanese.