Noun and の adjective
すいちゅう
水中でしか使えない。
It can only be used underwater.
右をみると、水中のせかいがひろがっていました。
When I looked to the right, there was an underwater world in front of me.
水中でめをあけることができません。
I can’t open my eyes underwater.
If you're in the middle of a bunch of water, then you're most likely underwater.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.