The Swimmer's Ring is a Magical Ring that Link acquires in Oracle of Seasons. The ring can only be collected in Oracle of Ages through the Ring Secret.
It is given to Link after completing the Mini-Game
Complete Master Diver's Mini-Game in a Linked Game
Increase swimming speed
"Swimming Speed +1"
In order to acquire this magical ring, Link must complete the Diver Secret. In a Linked Game of Oracle of Ages, one screen south of Jabu-Jabu's Belly on the surface, Link encounters Master Diver's Pupil. He is identical in appearance to the Master Diver from Oracle of Seasons. He gives Link a Secret and he asks him to tell it to the Master Diver in the Sunken City of Holodrum.
The Master Diver is found inside the building on the west side of the Sunken City. After completing a short mini-game for the Master Diver, Link acquires the Seed Ring that becomes the Swimmer's Ring when appraised.
When equipped, the Swimmer's Ring increases Link's speed while swimming. The use of the Swimmer's Ring is very limited in Oracle of Seasons. This is because there are very few areas where Link can swim. It has much more use in Oracle of Ages.