Kind of depends on the scale we're talking about. In 1/72 it would have to be very subtle or it'd look overdone and cartoonish. I tried to get something along those lines with my just-completed A6M2-N Rufe, using Polyscale's acrylic Dust, and got a very faint, but noticeable - in the right light - whitish line along the floats.
Another thing you have to consider is what kind of seaplanes are we talking about? Some big ones, like PBYs, for instance, were moored in the water at forward bases but pulled out on land at large bases, like Pearl Harbor; smaller seaplanes like Kingfishers were usually hauled back aboard their ship or up onto land when their patrol was done. In all those cases, the pontoons and hull areas that contacted salt water would be hosed off with fresh water as soon as possible. I am not a seaplane expert, but I believe, in general, that all the naval air forces preferred to keep their seaplanes out of the water as much as possible, for the simple fact that seawater is murderously corrosive.