Azurine Lake is the biggest body of water in the Reborn region. It's extremely polluted for a large part of the game, until the player shuts down Team Meteor's operations in the Mosswater Factory, Blacksteam Factory and Water Treatment Center.
Azurine Cave can be found on the northwest section of Apophyll Beach. Surf northwest until you reach a small pathway that bends to the right. A huge diving area can be found here. Dive and locate a small diving spot near the center to resurface to an enclosed area leading to the cave entrance.
The Diamond Ring is found on the southwestern house by the diving spot northwest of Apophyll Beach(K-Area). The Diamond Ring can be found on the bottom-left house underwater. Return the ring to the woman in Jasper Ward. This is part of the The Klefki Sidequest.