The uncharted island map is a map which can be purchased for 800,000 coins from the travelling merchant, or found randomly while fishing at the Deep Sea Fishing hub. The map leads to an island in the Uncharted Isles with 3-6 special resources, as opposed to islands with 3-5 special resources generated from long voyages or regular uncharted island maps.
Obtaining the map by fishing may rarely grant a red uncharted island map instead, which grants the player access to an island where Tavia's fishing rod can be obtained. There is also a chance to additionally receive the rarer red map when purchasing this map from the merchant.
Reading the map prompts the player to say "If I take this to my quartermaster in the Wushanko Isles, I can visit this island."