Red Reef Park in Boca Raton, Ocean Reef Park on Singer Island, and Coral Cove Park on Jupiter Island are fun, very manageable free dive sites.
Also Blue Heron Bridge in Riviera Beach. If you are just doing the snorkel trail, high tide doesn't matter nearly as much as if you are going under either east or west bridge span.
+1 on the Palm Beach County Reef Smart guide. It is an excellent source and can be downloaded on Kindle.
If you go to Red Reef Park in Boca Raton, be sure and visit Gumbo Limbo Nature Center. They have 4 awesome aquariums spotlighting the 4 key underwater ecosystems in South FL (mangroves/intracoastal, near shore reef, deep reef, artificial reef). I used to be an aquarium guide there. It was a blast!