Sunken Coins are an uncommon item solely used to pay off your debt to the Sea Treasure Hunter, as well as summoning rare treasure caches.
Sunken Coins can be found in both Sea Treasure Caches and Large Sea Treasure Caches. Prior to finishing the Sea Treasure Hunter's Quest, the Sunken Coins are used to pay off a debt to the Sea Treasure Hunter as he gives the player the Grabber, followed by the Sooner, which require 10 and 20 Sunken Coins respectively. Once the player has finished the quest, they can give the Sea Treasure Hunter 10 Sunken Coins in order to have him summon a rare treasure cache.