There are several treasure chests on and around the island. In the Water just north of the island, a chest can be brought up which contains a Giant Boomerang. A second chest is on the main island, stuck in the ground, and it contains a Silver Rupee. A third chest is found underwater, just south of the island and it contains a Silverscale Spear.
Nearby Korok Seeds
Complete the circle of rocks.
Swim to the flower trail and follow it.