Where can you go for a tropical beach eco-vacation that is not too hot and buggy in August?

August is the only time my friend and I can go. But Mexico looks really hot and the Caribbean sounds full of mosquitoes. Is that true? We live in upstate NY and do not have too much money- we can't go to Hawaii for example. We are not into big resorts and would prefer a quiet, uncrowded, eco-oriented, restful vacation. Any suggestions?