spend the money ! you have to take into account the very expensive taxi ride from international airport to water taxi terminal in Bz city and its a hassle all round. unless of course you arrive early in the day and want an adventure before dark and then... well it could be fun.
Here's a thought from another tourist. Elbert's right. $25 cab to the water taxi terminal. Undetermined wait at the terminal for the next boat. 1.25 hour ride to San Pedro. You're getting there late enough that I'd just fly.
I enjoy the water taxi to Caye Caulker but not the extra ride to Amberbris.
Just get on the flight from international to AC. Most flights leave the states really early and you will be too tired to check out the city and you see very little of it during the taxi ride. And it is so freshing to get here, you will love stepping off the plane to San Pedro.