You will have no problem getting a water taxi to Caye Caulker. The last water taxi departs at 5:30 p.m. Cab ride from the Phillip Goldson International Airport to the marine terminal is give or take $20 U.S. one way (for the cab), and one way water taxi from Bze. City to Caye Caulker is $9 U.S.
Hope this helps.
Cheers,
Wendy