Go to the small waterfall that’s available from Iza’s bomb arrow jar game. Go to the left of the waterfall and put on your Zora Armor. Take your Clawshot and Hold L-targeting while holding your Clawshot button. Swim into the wall and you should appear on the other side. You can then swim down to under the bottom of the lake. Just be sure not to pass the boundaries of the lake or else you will die.