The Ghostship is a ship that you can get on whilst travelling from Venore to Darashia using the boat. The chance of getting onto the ship is between 8%-13% (9 successes in 80 trials), so you can expect it will take less than 28 trips before you get to the ship. There's also a small possibility of getting on this ship by failing at mission 9 of the Shadows of Yalahar Quest.
To get off this ship, go down the ladder, follow the bridge south (kill some Ghouls and Skeletons), go up the ladder in the northeast corner and walk into the portal to be teleported to Darashia. A Ghost protects the portal; you can either kill it or ignore it.
There's also a little quest for you to do here.