Monokote is waterproof... except for the posibility of seam seepage. You can prevent the seam seepage by coating the seams with clear Lusterkote.

However... Monokote is relatively easy to scuff through when you taxi the plane up to te shore. And once you scuff through the mylar it certainly isn't waterproof anymore. For added protection, I'd pre-paint the hull with a couple of coats of thinned BalsaRite. (Thinned so it soaks into the wood better, making a thicker waterproof l;ayer without having to build up a heavy, all paint, layer)

Note... if the plane is planked over foam type construction... be careful with BalsaRite, it eats foam.