The watertaxis are normally ($15us/$30bz)one way.
There has been a special for (10us/$20bz)one way.
The taxi from International to the Belize City watertaxi is usually ($20us/$40bz), but I think they add for over 2 people.
For four people you can save some money but lose island vacation time. Less than four, take the plane. The view is worth it.