Our boat is also exclusively in freshwater now, Missouri River. We bought the boat in Newport, RI though and it had just had a very nice anti-fouling bottom job done in 2015. Our section of the Missouri is also heavily contaminated with zebra mussels but they don't seem to attach to hulls very much, coated or otherwise, or the aluminum pontoons for that matter, at least not around here. They do cover intake grates and screens really badly though, and the dock structures and if you leave a line trailing in the water for even a week the mussel buildup is astounding. They do build up badly on the bottom of our rubber (pvc) dingy, takes dynamite and laser beams to blast them off. I have been thinking of doing an anti-fouling coating on the dingy, as I recall they make a/f paint for pvc and hypalon. As for the boats in our marina though, anti-fouled or naked, we usually just get a thin slippery layer of river slime on the hull that comes off with a pressure washer pretty easily at the end of each season. (The post above that talks about zebra mussel build-up on hulls is interesting to me, doesn't seem to happen as much here, although they do build up like crazy on other surfaces. Maybe the slime layer on the hulls here makes them more inclined to attached to other surfaces instead.)