XTC can arange this for you as well at Tony 011-501-226-2884 on the San Pedro side. You will need a "Zarpa" from the Xcalak port master, which is the Immigration and Customs release form. You will need this if you want to legally enter Belize and receive an Immigration stamp. Although it is easy to circumvent the immigrataion process (just jump off the boat), it is highly illegal and you can face fines, jail time and deportation if in the unlikely event you are caught or get in an accident and need attention, etc...
We paid $300US on the last trip down from Xcalak (one way) at night!