Swimmer's Ring

From Zelda Wiki.org

The Swimmer's Ring is one of many Magic Rings obtainable in the Oracle Series. This particular one can be obtained through a Password-Linked sidequest. First, talk to a diver in the Present Age on the Zora Sea of Oracle of Ages to learn a password. Then take this password over to a diver in Sunken City of Seasons and pass his brief mini-game to earn the Swimmer's Ring.

For the amount of trouble Link must face to earn this ring, it doesn't do anything outstandingly useful. When worn, all this does is increase his swimming speed.