The Golden Scale is an item obtained in Ocarina of Time. Unlike the Silver Scale, the Golden Scale enables Link to dive even deeper into water before needing a breath of air. Although this item is very useful, it is not necessary to complete the game.
The Golden Scale is rewarded by the owner of the Fishing Hole. It can be obtained after catching a 13 pound fish or bigger. After getting said item, Link can then proceed to the Lakeside Laboratory and do a small side quest for the Lake Scientist. Link must utilize the power of the Golden Scale and dive to the depths of the pool inside the laboratory. If done correctly, the scientist will award Link with a Piece of Heart.