As long as you're past the safety issue...
From a purely cosmetic standpoint, 20% on a boat or open convertible appears as light as 35% on a hardtop vehicle since the interior isn't shaded. 35% just doesn't show up as much.
The reflection off the interior of darker film is particularly dangerous at night on vehicles with vertical glass. Since the boat glass is lower and curved I don't know that its a problem until something is illuminated down in the boat.