Go under

The boat was too heavy and went under.

In the crisis, the company was forced to reduce its costs in order to avoid going under.

If we don't get him to a doctor, he'll go under for sure.

Good for a fortnight, am I? The doctor told you? He lied. I shall go under by morning, and—— Put that nurse outside.

He goes under the name of Mr X to remain anonymous.

"But I went under nicely and I am glad there were some fine materializations."