From the marine terminal in Bz City, you can get a bus direct to Tikal for about $25 US. Leaves at 9:30Am & 3:30 PM (if it hasn't changed). Takes about 3 1/2 hours. Taxi from the muni to the marine terminal is about 5 minutes.
Mobunny - I don't really know of anyone who does the Tikal tour as an overnight from SP now that the airlines are not flying it. You could get yourself to Bz City on a water taxi and catch this bus. Not something I can book in advance. Maybe somebody on the island who has done it can help more. Or you can get yourself to San Ignacio and easily arrange a tour from there. I could get you a driver from Bz City to SI, or you can bus it. Spend a night or 3 in SI - it's a great town! LOTS to do and see.