The Treasure Hunt Island is a very small island (but has a large underground cave system) and is located south-east of Nargor, here. It is called Treasure Hunt Island because it's the final stage where the Treasure Hunt Quest takes place.
Captain Waverider can bring you to the island, but only when you are in a certain mission of the Treasure Hunt Quest. If you completed the quest you can't go back to this island. When saying "map" to Captain Waverider on Treasure Island he will respond with:
|Captain Waverider||Ferryman||✗||Liberty Bay (here), Treasure Island (here) and Treasure Hunt Island (here)|
|Polly||Guard||✗||Treasure Hunt Island|
|Skip||Guard||✗||Treasure Hunt Island|
|Skeleton Guard||Guard||✗||Treasure Hunt Island|