Sounds like Island has exactly what you want/need
From the book 'Belize Adventures in Nature' by Richard Mahler,1999
"Experienced kayakers can also follow the entire length of the barrier reef,camping along the way on such small islands as Bluefield Range,Spanish Lookout,Colson Caye,Half Moon Caye,and Northeast Caye "
Places also mentioned in that book and elsewhere as places to sea kayak: Glovers Reef,Lighthouse Reef,Chetumal Bay,Ranguana Caye,Hol Chan Channel,Caye Caulker,and Ambergris Caye (San Pedro)
There were a lot of blog people bitching about wind trying to access the reef from San Pedro,but I don't think they are knowledgeable about sea kayaking.