If you can still get sun screen in your area, just read the bottle, or whatever it comes in. If your area has already put it away for the season, wait until you get to the town where you are leaving from, and shop there.
You might try the organic or specialty section of your local grocer. I've had good luck there. Also, I think (you'll want to verify) the "Kiss My Face" products are 100% biodegradable; you can buy their products online.
Monarch of the Seas - December 2006
Elation - April 2006
Sensation - May 2005