I think this should also be a good start - other can help fill in the details perhaps?
Only comment is that if it is a long-boat as I know it doesn't have a lower deck to explore. A medieval Caravel or similar ship would provide more opportunity for the PCs to loose sight of their surroundings.
It also assumes that the PC's all pass below deck, which might not happen without GM railroading. Not saying it is not possible, but it would take a fairly naive party not to leave a lookout up top.
Perhaps it only shows up when the PC's have been shipwrecked and it represents the only means of escape. The PC's ship runs around on an island after a severe storm blows them off course. Their ship is unreparable, due to total absence of trees on the island, and insufficent materials to complete the repair. However, in scouring the small island, they find this ship moored in a sheltered cove. The ship springs its trap only once the entire party has come aboard and are trying to head home.
Now, where are they going?