To Sink Something
To Submerge Something
transitive verb, ichidan verb
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means sink so the verb vocab version is to sink something or to submerge something.
This is a transitive verb, meaning it takes a direct object. So instead of describing something sinking on its own, you use 沈める when you sink an enemy ship or submerge yourself in a bathtub.
You know this because you can't sink anything until you get that mail (める) saying, "Yes, proceed with sinking!" You've got to have that mail if you want to sink something, or else everyone will blame you for the missing ship — or for you disappearing under the water.
When you sink something, like a battleship, you go on and say: "she's (しず) sunk, I sunk her." That's like your tagline, or something. You say it for some reason because you think it's cool.
彼はソファに身を沈めながら、アカデミー賞の授賞式の生中継を観ています。
He is watching the broadcast of the Academy Awards ceremony and sinking into the sofa.