Defeat the captain and go to the secret island of the Captain
The treasure is on a secret island which is revealed after you find all pieces of the map, called Skullduggery. But I don't know how to get it yet.
if you are talking about treasure maps, they are used to find treasure...
There is a dock at the bottom side of the small island, Skullduggery. The island appears when you have collected all 5 of the pieces of the Treasure Map.
You can't find the treasure until you have the location from the Governor at Fort Ridley. When you have beaten Captain Crawfish, and have all five pieces of the map, see the governor to get the location card and the Bone Shovel. Return to Skullduggery Island to dig up the treasure.
Once you have all five pieces of the Treasure Map, the 6th island, Skullduggery, appears on the map. When you acquire a powerful ship, and scare off Captain Crawfish, the Governor will give you the Treasure location and a bone shovel to dig it up. (see related question)
the treasure map is fake and the pirates are a made up but, the pirate Fredric drake made all the treasure maps!
You can't find the treasure until you have the location from the Governor at Fort Ridley. When you have beaten Captain Crawfish, and have all five pieces of the map, see the governor to get the location card and the Bone Shovel. Return to Skullduggery Island to dig up the treasure.
Nothing until the very end of the game. Once you have all five pieces of the map, put them together to reveal the 6th island ("little Skullduggery"). You can go there but there is nothing to do. After you chase off Captain Crawfish in battle, the Governor at Fort Ridley will give you the location card for the Buried Treasure, and a bone shovel to dig with. Sail to Skullduggery and dig up the treasure.
The buried treasure is the final part of the island. It is on the hidden island (Skullduggery) but you can only get it with the Location Card and Bone Shovel that you get from the Governor. Once you combine all five map pieces, and defeat Captain Crawfish in battle, you can go to the Governor to get the exact location, then travel there to dig it up.
Once you have all 5 map pieces, the new island (Skullduggery) appears near the Pirate Outpost. But there is nothing you can do there until you acquire a Phoenix Warbird and defeat Captain Crawfish in battle. Once that is done, return to the governor in Fort Ridley and he will give you the map to the treasure on Skullduggery, and a shovel to dig it up.
Once you have all five, merge them together to reveal a 6th island (Skullduggery) at the bottom center of the map, near the Pirate Outpost. You cannot do anything there until you get the largest ship and can defeat Captain Crawfish. Once you defeat Captain Crawfish in battle, take the map to the Governor and he will give you the Buried Treasure location and the Bone Shovel. Now you can dig up the treasure on Skullduggery.
Once you defeat his ship, and have all five map pieces, return to Fort Ridley, where the Governor will give you a bone shovel and the location of the buried treasure. Sail to the new island, Skullduggery, and dig it up.