Once you have completed Lakebed Temple, go to Zora's Domain and swim to the bottom of the water there with your Zora Armor on. Go to the rock that smashed the ice to begin with, at the bottom, and about half way up it you will realise that you can target it. Go to the top of the rock, where the flat bit is, and use your iron boots to stand on it. Once you are stood on it, get out a water bomb and place it on the flat bit. Quickly move away from it before it blows up.
When it does blow up you will see a Goron burst out of it. Talk to him and he will tell you not to leave. Talk to him again and he will give you a second bomb bag containing 15 bombs.