This is a medium-sized island named for the giant stone face that sits at its top. What could that huge stone face be watching over?
Blue ChuChu
Defeat the Blue ChuChu behind the giant stone face for a Blue Chu Jelly.
Piece of Heart #18 (via Sea Platform)
Sail up to the single Sea Platform and destroy its cannons to reveal a chest containing Piece of Heart #18.
Triforce Shard #5 (via Secret Cave)
With the Power Bracelets, lift the huge stone face at the top of this island to reveal a hole. Drop into the hole to enter a small Secret Cave. Clear each room of enemies to unseal the final door. When you enter it, play the “Wind’s Requiem” at the Wind Crest on the floor to reveal a chest containing Triforce Shard #5.
Triforce Shard #4 (via Triforce Chart #2)
After you find, open, and pay Tingle 398 Rupees to decipher Triforce Chart #2 (found at the Bird’s Peak Rock [G5]), you can pull a chest from the Great Sea near Stone Watcher Island. The chest contains Triforce Shard #4.