Enjoy exploring one of the only undeveloped portions of Boca, right along the notoriously grubby Rio Canal. The The cache is pretty easy to find, and to reach by kayak or small boat. You'll need to leave your boat at the mouth of the lagoon, or pull it up onto the beach. It's a clear tupperware container with a paper and a Chewbacca pen inside.
Note: Contacted the city, they said it'd be OK, but that it would be considered tresspassing if you went between sundown and sunrise, so don't go during those hours because of that and because it'd be too hard to find anyway.