Placencia is 'way' in the south. Very different from San Pedro. I have been to Caye Caulker a few times, the less touristy and even more laid back version of Ambergris.
Took one charter there and got into a couple permit, as shown in my profile pic.
Literally caught bone fish from the dock of our bungalow WHILE MY WIFE WAS SWIMMING. They were everywhere. Even got a decent sized baracuda and fought a tarpon right from shore. (didn't land the Tarpon, my gear was outgunned).
Offshore is a different story, never tried that out of Belize. There is a barrier reef that you need to get past to get out to blue water.