The Enchiridion is a book for heroes in the Land of Ooo. If you are heroic enough to find your way to it, you may fairly sign your name therein, and join the heroes' ranks. To do this, you must first complete "Adventure Time" - a four-part Adventure Lab series that takes you into the same waters as the final (so they are best done together in a single outing). The difficulty rating here reflects the work of completing Adventure Time - This bonus cache should be easy to find once you reach the correct coordinates.
The coordinates listed above are for the put-in, which has a parking area and boat ramp. It's open 24/7, and since the Lab cache uses reflective tacks for clues, it can be completed day or night. The Lab can be completed without leaving your boat, but you will need to go ashore and walk a bit for the bonus. Good luck!