Ghost Ship is almost the same as Sky Ship http://www.nexusmods.com/fallout4/mods/15456/? just in different location and in the ocean.
Workbenches are linked with Castle workbench. You can enter directly in interior thru door next to the Castle entrance, or on deck using fast travel.

Update,

Added new version: Added small island, teleport boats...removed teleport doors from Castle, you must enter the boat first than to travel to the ship and than to enter in thru four doors by your choice. Boat is close to the Castle entrance, by the Castle port. You have all in pictures.