The chest's locked? What an unforeseeable twist. I am heartbroken.

It's that fishbone-looking thing hanging around Harm's neck.

Unfortunately not. We'll need him to take it off on his own.

It's a convoluted puzzle, but here's the basics:

By making Harm SUPER itchy, he'll have no choice but to take the key off; it'd make the itching worse, after all.

Itchy. Try checking the bathhouse - the woman there tended to injuries, so one of the patients must have had an itchy affliction.

Unfortunately, the bathwoman isn't willing to share the powder.

The bandages she uses are thrown into the sea, apparently.

But potentially useful - if we put something between the window and the water that could intercept the bandages, we'd have a lovely supply of... itchy bandages.

Something that could be hung up and keep the bandages dry inside of it.

You can find something like that inside of the bathhouse.

Grab the bucket sitting next to the window, then proceed outside.

You'll then need a hook to attach the bucket to the ring.

Where there's fishermen, there's hooks.

Try the docks.

There's a big row of them next to Harm's ship.

Grab one, fix it to the ring next to the bathhouse, then hang the bucket from the hook. You'll get a bandage soon after.

Harm didn't need bandages, but was apparently told to drink cod liver oil; you'll find some in his cabin.

Combining it with the itchy powder on the bandages will create quite the miracle cure.

See what Harm thinks of the taste.

You still can't just grab the key - we'll need to switch it for a fake.