This is the same island you'll visit briefly during the Hidden Tax side quest. The location objective is to loot the island's two treasures, but note that the one underwater by the docks does not count here. Instead use Senu to tag the two toward the eastern side of the island.
The inland one is beneath the distressed wood here- strike to break through.
The other is a red chest in the shelter by the shore, likely swarmed by crocodiles.
Claiming both completes the location.