Popular spot is accross river from shell pile at Bagdad on Blackwater. I have never tried it, but mullet were so thick at Grassy Point yesterday, I wished I had a big treble hook with me to give it a try.
Go to http://www.santarosa.fl.gov/parks/boatramp.html and click on Oyster Pile Boat Ramp. Then click on map. That will show you how to get there. Put in, go north to the mouth of Pond Creek. Turn East. Cross river to the eastern most route. You will probably see boats there who are snagging mullet.