I have never found a perfect, waterproof clearcoat. I really like the enamel based dullcote/flat clearcoat from Testors. However, it takes weeks to fully dry before it will get to a point where it doesn't form permanent white spots when submerged. More recently, I use Top Flite LustreKote Spray Flat Clear. I haven't observed any white spots, but it's not fully waterproof. After running my sub for about 45 minutes, I can see the Future basecoat (that I used for help with decal application) start to absorb water and get somewhat milky. Everything dries out no problem, though and no permanent white spots form.
I think next time I will use an enamel black color coat, then top it with the Top Flite LusterKote and not use Future. Yes, water will get through the LusterKote, but Testors colored enamle paints handle water just fine and both the enamel color coat and LusterKote will dry out no problem.