Go to where the small waterfall that is available from the Iza's bomb arrow jar game. Go to the left of the waterfall. Put on your Zora amor on take out your Clawshot. Hold L-targeting then hold the button your Clawshot is assigned to. Swim into the wall and you will be on the other side. You can then swim down and under the bottom of the lake. Do not go past the boundaries of the lake or you will fall and die.