From above, Horseshoe Island looks like what it’s named for. Use the Deku Leaf to knock Deku Nuts into holes to progress along the curve of the horseshoe. You need the Deku Leaf to traverse the island.
Silver Rupee (via Treasure Chart #9)
Find and open Treasure Chart #9 (inside the submarine near Crescent Moon Island in sector E1) to pull a chest from the Great Sea near Horseshoe Island. The chest contains a Silver Rupee.
Treasure Chart #28
Use the Deku Leaf to knock Deku Nuts into three holes here. This gets rid of walls of thorns that block your progress. At the end of the horseshoe, use the Deku Leaf to float to a chest on the island’s central rock platform and collect Treasure Chart #28.
Treasure Chart #8 (via Secret Cave)
Drop into a hole at the end of the horseshoe to enter a Secret Cave below. Defeat a Mothula and two Wingless Mothulas to reveal a chest containing Treasure Chart #8.
Treasure Chart #43 (via Sea Platforms)
Sail to a pair of Sea Platforms and climb either of their ladders. Step on a pressure switch on one platform to reveal a chest on the other. One chest contains a small reward; the other contains Treasure Chart #43.