Cancun- Guatemala boat trip

Hi,
I did a trip through Central America earlier this year, and have decided with my partner that we are going to Guatemala for our honeymoon in November 2008. we're going to be flying into Cancun and moving onto Guatemala from there, and while I know that we could hire a boat and crew in Cancun to sail down to Guatemala with, I was wondering if anyone knew how much that would cost, or if there were regular services from Cancun to either Livingston or Puerto Barrios?
Thanks for you help