So it turns out that inflatable dinghy anti-fouling does not entirely prevent barnacles. I put a double coat of it on about 4 months ago.  The dink has spent almost the entire time in the water since then.  Apparently there are no barnacles in the Keys so this has grown in the past 6 weeks or so.  There were maybe 100 or so 1/4 inch barnacles.  They came off easily with the handle of a plastic spoon. 

Not sure if I'll put it back in the water or not. Probably will do so for the next week to determine whether or not this anti- fouling is still basically effective or not.