People who want to do volunteer work in Belize can sometimes find such an opportunity by contacting the many non profit organizations who have projects there...several international environmental organizations (like Earthwatch for example), many religious groups (like Habitat for Humanity), etc. Such volunteer efforts are not glamorous, they are usually located in poor areas, and I would be very surprised indeed if any were on Ambergris Caye. This work is usually physical labor (like building homes) and the projects are temporary. I don't think this is what the young fellow from Texas had in mind, but if it is, he can do net searches for volunteer slots.
As for the "retired teacher," I don't know whether people who want to DONATE their services as teachers can get work permits to teach in Belize or not, but again, it would not likely be on Ambergris Caye. There are young Belizeans who can take such jobs (though they are hard to fill because the pay is so low...).
AmbergrisCaye.com 
