How can I get past the Captain?

First you need to convince him you're not a passenger.

Do you remember coming across a uniform you could use anywhere?

Evan had a lot of costumes in his travel trunk.

Go to room 3015 and get the uniform top.

There's somewhere else you've seen spare clothes, though.

Get the trousers from backstage in the Three Skills Club.

Show off your uniform to the Captain, and learn about his erratic sleeping patterns.

Sing the sea shanty to him once you have it.