If you go to the back of the boat on old school mode, and go up the stairs to the second deck you will find on either side a gap in the railing for the third deck.

Get a friend to crouch and look down in front of it.

Looking at your friend turn around and stand on the wall attached to the railing.

Run and jump on your friends back and jump up until you land be on the third deck.

To go in the room jump and keep taping the B button and you will go in.

Do the same thing on the way out.