Horseshoe Island (A7 on the map) is one of the 49 different islands featured in The Legend of Zelda: The Wind Waker for the GameCube. The name of the island derives from the fact that from above it resembles a horseshoe. In all there are four sidequests that the player can partake in here.
The first is found out in sea next to the island. You'll find two sea platforms, both next to each other. Each one has a switch, and pressing one will cause a treasure chest to appear on the other half of the pair. The treasures found within the chest are negligible. The second sidequests is also at sea, and in order to get it you'll need to have the 9th Treasure Chart. Using it will help you find a Silver Rupee out in the ocean.
On the island you'll be able to find two treasure charts. The first chart (#28) can be found by using your Deku Leaf to blow a nut into a wall of thorns, which will get rid of them. Continue up the island and use your Deku Leaf again to glide to a platform where you can claim your treasure. The other chart (#8) can be found by dropping down a hole and defeating the enemies within.