Rolling in the Deep is what a ship far out at sea does - in old fashioned, poetic language. The image is a small ship tossed on a very deep, dark ocean.
At the beginning of this song the singer says “Finally, I can see you crystal clear.
Go ahead and sell me out and I’ll lay your ship bare.” starting a nautical illusion. Later on she says “Think of me in the depths of your despair.
Making a home down there, as mine sure won’t be shared.”
The lyrics conjure for me an image of the man alone in a deep sea of despair like a lonely ship rolling in the waves of the deep ocean - Rolling in the Deep