Each time you rock the boat you have a very small chance of getting a "map piece" When you get all 4 pieces of the map, you get 3 prizes! The first and coolest is the Mermonkey artifact. It looks like a golden trophy! Then you get a treasure chest and finally a gold dabloon. The only problem with the dabloon is that we have not figured out what it does yet. Besides sit in your dock and look pretty! LOL! Let me go see if I can find a link to the thread that explains it and shows you pictures!
It took me a couple of months to win a map piece four times to get it, so hang in there and keep trying!