I believe the island he is referring to is Iron Island. And actually, it's one of the islands you absolutely must visit in order to complete the game. You acquire a Triforce Chart here after you destroy the cannons and get inside the actual tower/island.
As previously stated, every island holds something. There are 41 Treasure Charts total in the game, and 8 Triforce Charts, making a total of 49 that you can pull stuff up from. The Great Sea in Wind Waker is laid out in a 7 x 7 grid, making 49 total islands. Each Chart leads to a treasure on one specific island, and no chart leads to a treasure on the same island as another one does.