My favorite Cause here on Ambergris Caye is SAGA because it helps those that can't help themselves. The way you treat your animals as a society is a direct reflection on how you treat each other as human beings.
Animals on the island are simply disposable when they are not cute anymore nobody cares about them and too many people (Parents) are not responsible enough to make the decisions needed to keep them healthy and happy.
SAGA has a great track record and has been doing this work for many years. They are also responsible for the fact that there is a Vet on the island.
If you love animals then this is the cause you need to contribute and assist.
If Children is your thing then there are several schools and the library that can always use help both financially and physically.
What I wanted to say in my earlier post was that whatever cause you decide to throw yourself on do it with the passion and focus. Trying to do too much will basically accomplish less.
Also don't just give people money without being sure what those funds will contribute to. Take an interest and don't be afraid to give an opinion but also keep in mind that most of these organizations are run by intelligent people who are doing what they can with what they have and in spite of everything going on around them (culture issues, government and critics).
Good luck on your quest