There is a new 4:30pm water taxi that goes from Belize to SP express (no stop in Caye Caulker). It is 45-50ft, totally covered, and has plenty of room for your group. It is the largest boat in the fleet and thusly is less prone to rough rides. Also, this is a "behind the reef" ride rather than an open ocean ride, so in general, the ride is pretty smooth. With this size group I would pre-arrange for taxi(s) to meet you at International. PM me if you want the name of my regular driver that could definitely handle this.