There are actually two ways to get from New Jersey to Belize - one is to go via Mexico, the other is to go via Florida.
The January 2004 issue of "Rider" magazine printed a very interesting article (pages 69-70) about the ferry that runs from Tampa, Florida to Merida, which is a town at the north-west corner of the Yucatan penninsula. It seems that motorcycles are quite welcome for transport on this ferry. Kind of helps that the owner of the ferry, the Chairman of the company, and the C.O.O. of the company are all riders.
The ferry itself is pretty reliable, by Central American standards - it is American registered and crewed, M/S Scotia Prince, and it normally operates in the summertime between Portland, Maine and Yarmouth, Nova Scotia.
More info here:
http://www.yucatanexpress.com/route.html
http://www.scotiaprince.com/index.php
Michael