It's "Key" and always has been pronounced Key since early British Honduras days. Keep in mind that Belize is a former British colony, and the only offical language is English.
Now if you want to get into Spanish pronunciations, that's another matter. In which case, let's all refer to Belize as Belice (Bay-Liss-Say). True, Spanish is becoming more and more important in Belize, given all the mostly illegal immigration from Guatemala and elsewhere. And Ambergris (Am-BUR-Griss), not the proper French pronunciation, was settled mostly by Spanish-speaking folks from Mexico. But IMHO Belize will no longer be Belize when Spanish is the dominant language.
As to the original question, it's always a good idea to have a back-up plan for getting more cash. But with 10 Belize Bank ATMs across the country now accepting ATM cards on the PLUS and CIRRUS networks, you (probably) won't have any trouble. If you do, you can always swipe your Visa or MC at most any bank ATM and get a cash advance, or failing that fill out the paperwork at the bank for the advance. But then the prudent traveler always carries a traveler's check (make that traveller's cheque or el cheque de viajero) if all else fails.