The clues to the use of the model ship as a treasure map lie within the main sail. There are three methods that could be used:
1- There is a code or pattern plainly visible on the sail which needs to be deciphered.
2- There is a code of pattern hidden on the sail, such as with temperature dependent ink (hold near a candle flame to view) or invisible ink that can be seen through a special glass or crystal.
3- There is a hidden message contained within the sail, a slip of paper or linen woven or concealed between two larger pieces that together constitute the sail.
I have tentatively gone with the temperature-dependent ink revealed by the heat of a candle's flame. The hidden message is something along the lines (no attempt at refinement or rhyme having been made):
"A true captain of the sea does not rely on a map in hiding his treasure.
He needs only the hull of his ship and his sails to lead the way."
The hull of the ship is made from knotted wood. Around the hole for the anchor chain is an eight-pointed star corresponding to a compass. When the ship is turned such that the SW arrow points north, the knots in the wood correspond to the islands visible during the monsoon floods. Between the knots is an almost imperceptible indentation in the shape of an X. This is the location of the sunken ship.
Unfortunately for those who seek the treasure, the knots do not correspond to permanent islands but, rather, mountains. So as they pore through nautical maps to find a match, they will be sorely tested.
Edit: I am adding my additional thoughts to this thread post publishing because I want to keep them all in the same place.
Problems I encountered while thinking of this:
1- How do the players know where to look? The ship sank ages ago in a desert covered by water. Their only map details a landscape entirely unfamiliar to them or anyone else because the valley has not been flooded more than a couple times since (and likely not even that).
Obviously even the survivor who made the model ship needed a prominent landmark on a continent-wide scale to find his way back to the ship lost in the ocean. Since his ship wrecked and he had to drift back to land, the map would allow him to find the place he came ashore and then lead to the treasure from there.
2- How to indicate such a landmark on the ship? A huge knot in the wood could indicate a nearby town or island or even continental coastline. But there needs to be a clue for the player's to discover its identity. It may be that the shipmaker did not intend for the model to lead himself back to the ship but, rather, for an acquaintance or descendant to find it.
The ship's name could correspond to the landmark via code or local nickname. Or the riddle on the sail could include the starting location and its indication on the hull. I had an idea a while back that the sail would have a spot in the cloth or a hole or the rigging would have a stray knot or two such that when laid over the hull it would indicate a particular knot or mark in the wood. This spot would be the location of the sunken ship. It could also be the starting location or a clue to it.
Another idea I had, in conjunction with the ship's name or sail containing the starting location (which would match up to the source material, since they were given the latitude and longitude of a town from which to begin their search), I thought of using the diary of the survivor. It, or fragments of it, would be hidden away in the town inside a chest or secret compartment. Inside the ship, either glued inside the hull or concealed within the mast (or consisting of the mast itself, after pulling it from the model), would be a key that opens the lock. The diary could then provide further details to find the ship.
But to stay with the point of the adventure, the real treasure of the sunken ship is not the meager gold it carries from its first foray but the half of the treasure map still stowed with its cargo. One wonders why the survivor did not put that map in his diary, be it in fact or by reproduction, so I want to steer away from the diary.