The small island (Skullduggery Island) appears once you have all of the map pieces, and view them together. However, you can't do anything there until you have beaten Captain Crawfish. Once you have, see the governor to get the location of the treasure. You can then travel to the island 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.
Collect the five map pieces (one on each of the trading islands) and join them to see the sixth island, Skullduggery. Once it appears on the map, you can actually go there, but nothing happens there until the end of the game. (see the related question)
Joining the map pieces reveals a new small island named Skullduggery, just to the right of the Pirate Outpost. However, there is nothing to do there until you defeat Captain Crawfish. Once he is beaten, go back to the governor and he will give you the treasure location, and you can go to Skullduggery to dig it up.
The 5 pieces of the treasure map reveal another small island (Skullduggery) where a secret treasure is buried. Once you get rid of Captain Crawfish, it will be safe for the villagers to recover it, and the Governor will send you to dig it up.
There is nothing to do on the small island, also called Skullduggery Island, until you have defeated Captain Crawfish. Once he is beaten, go back to the governor and get the map to the treasure, and a bone shovel. Return to the small island and follow the directions to find the buried treasure.
Skullduggery Island.
there is actually pieces of the map that you get on the ports but there is a puzzle you have to do to get them
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.
Although you can go there and look around, there is nothing to do until the end of the quest, once you have driven off Captain Crawfish and the Governor gives you the location of the Buried Treasure on Skullduggery.
The bank on Skullduggery Island is the Golden Harbor Bank, located in the Arabic-themed port at the lower right on the island map.
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.
Nothing. Captain Crawfish was there. But you don't exactly do anything on Skullduggery island. You defeat Crawfih on sea.