.. How do I get on the ghost ship?

The ghosts on deck can see you.

If only you were invisible to them.

Do you remember what the cannibals said about the head of the navigator?

Its necklace makes it invisible to ghosts.

Maybe you could use its necklace.

Try talking to the head.

It won't let you use its necklace without some persuasion...

And some threats or begging.

Use dialogue option [2] and then keep using dialogue option [2] or [3] until the head of the navigator agrees to give you its necklace.

Now use the necklace.

You can now walk around the ghost ship freely.