Overlook Island

Move your boat close to the rock pillar with the palm tree on top. Target the tree with your Hookshot and pull yourself up as soon as the waves carry you within reach.

Use the Hookshot again and fight a blue ChuChu. Make your way to the last cliff and you will see a hole in the ground (you can go higher, but there are only some pots up there). Jump into the hole.

Inside, you will find a battle dungeon. Whenever you clear a room, the torch on top will be lit.

Clear all the rooms by defeating the monsters (don't forget to steal Joy Pendants from the Green Bokoblins and Knight's Crests from the Darknuts). Once the dungeon is free of enemies, the door with the Triforce symbol above it will open.

Enter and stand on top of the Wind Crest. Play the Wind's Requiem to make a treasure chest appear. Inside, the Triforce Shard 8 (Wii U) or the Triforce Chart 8 (GameCube).

Once you're back outside, don't forget to look for the hidden treasure close to the island.