Flight... after going through Customs, walk several metres to the domestic flight check-in and get the next flight. The flight is about 15 minutes. Book tickets in advance to reduce any possible waiting time. Easy.

Water taxi... exit the airport and get a taxi to Belize City. If your group is more then 4 passengers, you'll need more taxis unless you can find a taxi van. The taxi ride is about 25 minutes, and the fare is BZ$60 (US$30) per taxi. Check-in for the next water taxi and wait. The water taxi trip is about 80 to 90 minutes. I've heard Caribbean Sprinter boats are a bit faster than San Pedro Belize Express. Check their websites for schedules, fares and trip times.