If you enjoy a warm, friendly, safe, laid-back location; then you'll LOVE Ambergris Caye. We first visited and completed our open water certs last year. 10 days was not long enough. We returned for another 10 days in November, and will return next November for 2 weeks. There are plenty of threads on the board recommending places to eat and stay.
As for diving, I HIGHLY recommend Amigos Del Mar. Great crew - very friendly and professional. This time, they even rinsed and stored our gear for us, so we wouldn't have to schlep it back and forth.
Get ready to be awed by the diving. (Feel free to take a look at the pictures my husband took on our web site at http://www.angrypig.com/belize05
Warm waters, can be rough on the surface during local dives, but just amazing once you're down. We were fortunate enough to see a ten-foot hammerhead during this last trip. No picture of him, though
Enjoy your trip...the worst part about visiting San Pedro is having to leave.