Open up Sharkfood Island using the codes for the secret items (I'll get those up soon...). Now, go to Treasure Trove Cove and swim out to the crate floating by Sharkfood Island and kill the shark. Jump in the water and swim to the bottom of the water. Traveling around the base of Sharkfood Island, you should find a hole in the bottom. Go in and you will be inside the Sharkfood Island polygon but not inside the Island itself. To get out, just swim back under it or jump onto the piece of land.
Submitted by: Mike