So I swam out to this little island and placed a cache. It's above the water line. Once you're there, look for a rock pile in the nook of the big rock. Please re-hide with lots of rocks so it doesn't get swept away.
After reading summer and winter logs, with amusement and satisfaction, I've adjusted the terrain and difficulty to a permanent 2.5/3.5. Turns out the winter finds are pretty tricky. More's the fun for everyone!