(Just found this post when googling "geocaching cruising.")
We are sailing out of San Juan a week later and have found several cachers on the roll call at Cruise Critic. Go to cruisecritic.com, set up a login, and then find the roll call for your sailing: Search something like "Golden Princess roll call" and then look for your specific date. Once there, there's an option to "Search this thread," and if you put in "geocach" you may find that some have already identified themselves as cachers. If not, just chime in with the question. (In three cruises, this is the first time any other cachers have turned up on the roll call, but this time I found three other couples plus several who are interested in meeting us to learn about it. We'll be meeting onboard the day we board.)
Good luck!