You can fish for this item before you start this part of the quest: Say "Mud fish" to Wim, the fishing NPC found in the curve of the river near Buya's east gate. It is best to use the 1000 coin rod and 500 coin bait options until you catch a Mud fish. It is rumored that fishing as close to midnight (00 hour) will increase your chances of catching this elusive fish.