If that is your itinerary, you will not be able to ride on the minibus that leaves from the water taxi terminal. The license of those minibuses only allow them to drop off and pick up people at designated spots in the country. There are, however, several places in San Ignacio where it is easy enough to get a minibus to Tikal. You might want to stop in at Eva's Restaurant on Burns Avenue. Bob can find you reliable transport to Tikal.