Surely Antifouling doesn't provide an osmosis barrier coat, for that you need to use an epoxy based barrier coat like Gelshield or West System Epoxy.
And I was always told that if you launch/recover the boat there isn't a need for a barrier coat, but it is highly recommended if you leave the boat in the water for more than a couple of months.
Conventional single-pack anti-foul paints are porous, and provide no protection other than (hopefully!) against marine growth. For osmosis protection you must apply several coats (500 mircons as a minimum) of two-pack epoxy resin.
Such an epoxy system is recommend for boats that are afloat most of the time, but is not necessary on hulls that spend much of the year out of the water.
