Oh look, another disguise. I'm not very surprised, but it's a relatively minor part of the story and so fairly unobjectionable. I have to admit, when the old man said he'd been out of work sick for a bit, I suspected that it was him - doesn't seem like much of an alibi if there's no one there to vouch for you. Although a large part of me did wonder if he'd disguised someone else as himself in order to fool a doctor and so give a little bit of support to the sickness story.

Overall, it's okay. Nothing astonishing, and it's not very exciting when the solution hinges on the existence of a ship that the reader doesn't know about until Poirot mentions it. Maybe back in the day this was common knowledge?