You have to take the boat with the black sails. It docks at various points on the coastline of the nothern realms, one spot I remember is right next to the tavern.
Oh! I see! Before that, I've tried a ship that goes along the route Slime -> Cats -> Frozen Island. But this one, with the black sails, somehow, was always hidden from my eyes. Now I just sat down and waited outside the tavern for about 5 minutes and... it really came! Thanks!
