A little in the way of explanation is called for I guess.
Ice on any lift-generating surface is a BAD thing. The reasons are manifold, but the main two are the addition of weight, and the change of the aerofoil shape. Even a fairly thin layer of ice can cause quite substantial changes in the lifting ability of an aerofoil section, and most often this will be in the form of a reduction in available lift. Add to that the extra weight of ice and you are looking at a dangerous situation, especially when attempting a take-off close to MTOW.
Additinally, if the ice accretion on each wing, or even along the length of one wing, is uneven then you can end up with a seriously off balance aircraft.
The problem with ice usually has very little to do with form drag due to an uneven surface, and everything to do with parasite drag caused by an inefficient aerofoil shape.
It's rather simple really. I wouldn't attempt to fly an aircraft with any ice on any surface.
As for using hot water to de-ice, that's not a good idea. If you've ever seen the effect on an iced up car windscreen of pouring hot water on you'll understand the problem. Differential expansion of the heated side can cause huge internal tensions to be created. The effect on glass in such a situation is often that the forces become too great and it cracks or shatters. Now the same effect can happen with aircraft skins, granted to a lesser degree due to the better thermal conductivity of the metals, but it happens nonetheless. Aircraft de-icing on the ground is carried out using a de-icing fluid, which consists of a chemical that causes the freezing point of the water to drop, thus preventing re-icing. Even then there is a limited "hold-over" time before a re-application is necessary.