On the main overworld map (Inkwell Isle I), look to the far right side. You will see a dock.
If the Boatman refuses to take you, it means one of two things: how to get cuphead the delicious last course
Once you reach Inkwell Isle IV, you will meet a fortune teller. She gives you a quest. You must find the (a hidden charm on Isle 4). On the main overworld map (Inkwell Isle I),
Here is the step-by-step to actually unlock the DLC content: how to get cuphead the delicious last course
After clearing a Mausoleum, a new NPC (the Boatman/Fairyman) will appear by the shore.