As a child go to Zora's Domain. Go to the top of the waterfall and pay to play the diving game. When the game starts, jump off the waterfall, swim over to the area of land in the water next to the cave and climb onto it. Wait here, facing towards the cave, while the timer counts done. When the timer reaches 00:01, jump off the platform towards the cave. As you are in midair above the water, time will run out, the Zora's message about you failing will come on screen, and Link will drop like a rock down through the water to land on the bottom right next to the cave. Press 'A' to get rid of the text and quickly swim to your right and into the cave.
Submitted by: Samuel