I know i have seen this asked somewhere else before, and i am pretty certain that the only way to get a realistic looking result was to model the bubbles, what i can't remember was if they had to be boolean cut from the solid ball or if you could just get away with them free floating. A little experimenting would soon reveal which it is. Then because everything is sphere based, you just rotate around the vertical axis to produce a pseudo random effect.
Hope that helps
Martin