If you're travelling from the International airport to CC, I'd say the easiest
way is by domestic flight from the International airport -- get off the international flight, go through Immigration & Customs, and walk a few metres to check-in for a domestic flight. Check the websites of Tropic Air and Maya Island Air for schedules & fares.
If you choose the water taxi, six people will need two taxi sedans, but look around for a taxi van which should be big enough for everyone.
Another option is Tropic Ferry. Tropic Ferry is more of a private service that has to be arranged in advance. Passengers are met at the airport, driven in a private a/c vehicle to the boat (a 3 minute drive to a marina located across the highway from the airport), and transported directly to destinations on Ambergris Caye. The price is higher than a public water taxi.http://www.tropicferry.com/