After taking the overnight bus from Mexico City and crossing the border, we paid our 300 pesos to leave the country (you can also pay in American Dollars and their is a bank at the border) and arrived at the Belize City bus station to be greeted by a load of taxi and tour guides trying to sell us tickets to Caye Caulker.
Travel Note: if you're looking to go to Caye Caulker straight away then these guys are your best bet. Negotiate with them on a good price to the water taxi stop - this is a 5-10 minute ride away so don't pay any more than 7-10 Belize dollar each. They will also give you prices in American dollars, but again ask or it in Belize as you will get a better rate.