Try these sites for some locations: http://snorkeling.jp/fun/point/map.php (Japanese, just move the map and zoom in on Okinawa. You can click on individual sites and get more info, Japanese, of course.)
http://www.mccsokinawa.com/mccs.asp?id=563
Read the info on this site for snorkeling basics and safety:
http://www.snorkeling.info/snorkeling-basics/index.html
I would also invest in some felt-soled dive boots for walking on the coral during low tide. Watch out for the sea urchins, stone fish, etc. Don't snorkel if the wind and waves are high. Watch the currents.
Check out this website for the marine animals you need to look out for (about a third of the way down the left sidebar):
http://www.virtualginza.com/okinawa.htm?japan3.htm
Have fun!!