Silver Scale

From Zelda Wiki.org

The Silver Scale is an item from Ocarina of Time that allows Link to dive deeper underwater.

In Ocarina of Time, Link is only able to swim downwards for a specific amount of time (or depth) before he is forced to surface. At the beginning of the game, this time is limited to three seconds (or three meters). The Silver Scale extends this time, allowing Link to dive six meters underwater (or, equivalently, swim downwards for six seconds).

The Silver Scale is obtained in Zora's Domain as an award for completing the diving minigame for the Zora on top of the waterfall. It costs 20 rupees to enter, and Link is required to collect the rupees that the Zora throws down into the pool below.

A more powerful (but optional) item known as the Golden Scale allows Link to dive 9 meters underwater.