Daniel,
Don't let these responses get you down! There are lots of adventure jobs out there! However, these guys and gals are correct.
As a traveling student myself, I went down during winter break. This was my 4th time down and I was participating in one of these volunteer projects in the northern city of San Ignacio (mainland). That was through the Cornerstone Foundation. You can check it out on the web.(There is also one called ProBelize.)
While I was there I paid for(he carried my bags in exchange!) a young male friend of mine, 19 years old and partially educated, from a rural village in the south who was looking for work. I paid his way to the Cayes to look for work with his uncle. Again, its the issue that it is a developing country so there arent that many jobs available.
There are sometimes positions for teachers as long as it is in a field where they dont have anyone qualified. Areas like computer science I think. But even so there is no guarantee and you have to have lived in country for 6 months first. There is also an apparent shortage of nurses. And if you do get a position you should think about training a Belizean to take your place!
An imortant point to remember for any foreigner going there for work or projects is that the people should benefit too. I mean if you are going to take a job or start a project think if a Belizean is capable of doing that and are you taking that away from them. Or can you get them involved somehow.
A good link for travel work is: goabroad.com.
The other option would be something like peace corps but who knows where you would get placed.
Hope this helps!
There are many sites describing all this that are already out there.