The last water taxi departs at 5:30 p.m., not 5:00 p.m. You will have plenty of time to clear customs and immigration and catch the last boat out.
It could be anywhere from a 20 minute to a 30 minute drive from the Phillip Goldson International Airport into Belize City, which is where both the Municipal Airstrip and the water taxis are located. Even though the drive is only ten miles long, this is so because the road is a two lane road which can become highly congested with traffic at times, especiall as you get closer into Belize City.
Once you get into Belize City, the water taxi terminal and the Municipal airstrip are no more than five minutes from each other by taxi.
Water taxi ride is 45 minutes long. Flight is 7 minutes long.