It's been sitting in my inventory for around a month or less, but what does it do? -.-

If you toss it in the ocean you can have access to port plunder if your a Islander or you can just pay tribute to Amy

Click on it, and you'll see an option to 'toss into the water' or something along those lines. Once you click that, you will gain access to port plunder.

http://www.subeta.net/explore/port.php c: