The narrator explains how the circumstances of his night aboard the boat paralleled the circumstances of his worst fears. What are the similarities that he experiences?

The question is incomplete, but the user is probably referring to the question about fear where the character from Poe's fiction is mentioned.

Explanation:

There is a similarity between the ship and the coffin.

During his sleep in the berth, he felt like he was lying in the coffin.

Furthermore, berths are often in the lower part of the ship which can give a certain feeling of being underwater which the character probably sees as same as being under the ground.