I like to seal the forward and rear end of each pontoon with a water-based paint.
This is because (a) it's an ideal means of de-stressing and (b) the spray-paint we'll be using would otherwise dissolve the foam.
Polyester resin does so too, though if it floats your boat then epoxy resin is inert in this respect.
It costs more than champagne, however, while not tasting nearly so good.
