The Servant's Ring is a treasure item in Endless Ocean 2. It is not counted as an actual salvage item, and is instead counted as a quest item and placed in the player's bag.
This unique item can be found in the Treasure Vault of Valka Castle, on a shelf near the back corner of the room, at coordinates A/B-1. It is not found using the Multisensor, but instead shows up as a sparkling, swirling patch in the water, like the one the player encounters when observing a quest item.
Upon returning to Nineball Island after finding the Ring, GG will pull the player aside and tell them about the ring they're wearing: the gold it's made of comes from the Cortica River, and was probably made back when people actually panned for gold in Cortica.
The next time the player is at Spirit Falls, a cutscene will activate, during which the resident piraiba catfish will swim up to the diver and beg for the ring. They player gets a choice, either give the catfish the ring, or don't. If the player selects not to, it just gets mad, and it asks again, so the "don't" option doesn't really do the player any good. When you give it the ring, it goes back to its normal behavior, until the player leaves the water, then returns to the same area. At that point, the normal catfish will have been replaced by the legendary Golden Catfish.