Flippers

Flippers, also known as the Zora's Flippers in A Link to the Past and the Oracle series, is an item that allows Link to swim. It first appeared in A Link to the Past and returned in Link's Awakening, Oracle of Ages, Oracle of Seasons, and The Minish Cap.

A Link to the Past
In A Link to the Past, Zora's Flippers are bought from King Zora for 500 Rupees and are required to reach certain areas of the game. They allow Link to use the Whirlpool Waterways to warp between bodies of water. Link can obtain them once he obtains the Power Glove.

In the Game Boy Advance remake, Link is able to dive underwater while swimming, much like in later games. This can be used to avoid attacks or certain traps.

Link's Awakening
In Link's Awakening, the Flippers are not purchased, but instead obtained from the Angler's Tunnel. The Flippers allow Link to dive underwater, an action that is necessary to enter the Catfish's Maw and to obtain various Pieces of Heart. By diving underwater, Link can avoid attacks from Zoras and other enemies while swimming (when he can't use weapons).

Oracle of Ages and Oracle of Seasons
In Oracle of Ages, the Zora's Flippers are found in Cheval's Tomb in Yoll Graveyard. They are used to pass Cheval's tests in order to get Cheval Rope. Later in the game, Link can find the Mermaid Suit, an upgrade for the Flippers, in the Mermaid's Cave. With the Zora's Flippers, Link can dive for a short while (if he is wearing the Zora Ring, he can dive for an infinite length of time).

In Oracle of Seasons, the Zora's Flippers are rewarded to Link in Sunken City if he can find the Master's Plaque and bring it back to the Master Diver with the help of Dimitri.

The Minish Cap
In The Minish Cap, Link finds the Flippers as a Minish in a sewer underneath Hyrule Town's library after a long sidequest which involves returning overdue books. Librari, a Minish elder, sends Link down into the sewer. With these flippers, Link is able to gain entrance to the Temple of Droplets at Lake Hylia.

Twilight Princess
While not specifically mentioned or named, the Zora Armor includes flipper-like boots that are likely intended to be Zora's Flippers. These allow Link to swim while underwater.