Hi,

I agree with the previous messagers, the bus is best, preferably the express bus to Belize City - it's very cheap and only takes about 2 hours. It's also an interesting ride as you get to see a lot of the culture and people going about their daily business. I recommend catching the water taxi as it also is good fun and very cheap but if you can afford it, you might want to fly one leg of the journey. It'll only cost you about $30 USD one way from the Municipal Airport (Belize City) and only takes about 15 minutes, plus you get a spectacular low level view over the sea to the Caye. You can even sit next to the pilot if you ask nicely which makes it even more exciting. Check out the two airlines that do the trip below. Don't fly to or from the International airport to the Cayes though as you'll pay twice as much as from the Muncicipal which is actually in Belize City and about a $10 BZD taxi ride from the bus station.
http://www.tropicair.com/ http://www.mayaairways.com/