It is much easier to arrange with the dive shop in Xcalak who come here (during busy periods)at least 3 x a week.The dive shop is run by two Canadians who are great fun and they will help you with leaving Mexico and arriving in San Pedro.
It is the cheapest and easiest way(when you get to Xcalak you will understand why)as just the drive to Chetumal is 3 hours.
To Tikal the cheap way is to take a water taxi to Belize city -catch a minibus to Flores (20usd)and spend the next day in Tikal.The following day reverse the trip only problem you have to be up at 5.00am