Yes, Amex covers CDW in Belize (though it may not cover all types of vehicles or in certain areas, such as off-road conditions -- call Amex BEFORE you get to Belize to confirm these exceptions).
If your car rental company REQUIRES you to buy CDW, find another rental company. Budget Belize, for example, is a good company that doesn't require you to take CDW. (If you buy CDW, Amex and other card coverage doesn't apply.)
--Lan Sluder www.belizefirst.com