Weevil

To breathe underwater, you need the Zora Tunic. You can get one for free in you unfreeze King Zora. To do this, you need to drop a Blue Flame from inside a bottle next to him.
Head to the place where Lord Jabu-Jabu was, jump across the floating ice, and enter the Ice Cavern. North of the main room (with the big spinning ice blade) is a room with a Blue Flame, which you can put in a bottle. There is also a Blue Flame in the room to the right of the main room, but you need to melt the red ice wall first.
After you unfreeze King Zora, talk to him and he'll give you the Zora Tunic.

You obtain the Fairy Bow in the Forest Temple. You'll need it to hunt down the Poe Sisters, who are hiding in portraits. You'll need the Fairy Bow to activate switches from a distance, such as the eye switches.