go below—(Nautical) go to the lower (underwater) levels in a ship:
- Nora and I went below to our bunks to make our beds and settle in before the ship left the channel.
go under—
1. (of a person or ship) go below the surface of the sea:
- Our boat went under in no time at all. Some time later the submarine approached us, spraying machine gun tracer bullets….
2. (of a business) fail:
- A few weeks before the company went under he checked to see if his pension was safe.
3. lose consciousness under the action of anesthesia, etc.:
- No one trained in administering anesthesia was dedicated to that task alone in his operating room as she went under.
See also: come down / go down.