Weider Fruit Overload has been developed to be mixed with water. As milk can bloat the skin up in the definition phase, many bodybuilders mix their shakes with water. It's possible to mix protein with water but the taste isn't the best.
The fruity taste of Fruit Overload leads the muscles to become overloaded with protein. When dieting, it's important to flood the muscles with amino acids, otherwise you could lose muscle mass. The whey protein isolate is split into amino acids quickly and can therefore flood into the muscles almost immediately.