Geron's an awful liar, so we'll need to find a valid reason to be on the ship.
Like we're... delivering something.
No, not like pizza. Think about it - when you're away from home for a while, what do you always come back to?
Well, yes, but you usually find lots of mail waiting for you. The honest merchant spending time at sea is no exception.
Find something that Harm was having delivered, and see if you can deliver it yourself.
The bathhouse seemed to be a fairly popular spot - someone in there may prove useful.
The injured woman mentioned something about delivering messages - she'll be worth speaking to.
Nothing particularly useful, but she's confirmed that she is a messenger.
We also know that she's wounded, and as such can't move around very much.
It's a safe bet to assume that her backpack is full of messages; one may be for Harm.
This town thrives on information; try asking the people in the bathhouse. One of them is bound to know.
The man in the tub closest to the messenger is actually her "boyfriend".
He confirms that the letter is for Harm.
We now know of a possible way on to the ship - next up, stealing the letter.