Once you have raised the water around Rusty Bucket Bay for the second time, go back to the first chamber (the one with the floating bomb) and hit the switch there to raise the water level to it's highest. Quickly swim back to the Rusty Bucket Bay entrance chamber and swim into the the hole in the wall. Go up the stairs and speak to Cheato, who will tell you to enter the code ''GOLDFEATHERS'' into the Treasure Trove Cove sandcastle floor. Do so and your maximum Gold Feather total will be 20!