Fix a flat is made to temporarily plug a hole in your tire, not balance it. And water in a tire is just a stupid idea, water freezes.
I know for a fact that Michelin uses all different sizes of rubber ***** in their high performance tires to help balance them, and IIRC Pirelli and Continental use them as well. So there certainly is some truth to using this type of product to balance a tire. They are however, not specifically intended to be used as a replacement for a real balancing.