Underwater
noun, の adjective
If you're in the middle of a bunch of water, then you're most likely underwater.
すいちゅう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
水中にコイがたくさんいる。
There are a lot of koi fish in the water.
このカメラは、水中カメラです。
This camera is an underwater camera.
水中マージャンやらない?
Do you want to play underwater mahjong with me?