Not to speak for Redsy but what he is saying is that # 2 al can be fused at 100 amps if it is a service entrance conductor or a feeder that supplies the dwelling unit main panel. This does not apply in the case as shown above. With out looking I think the table is 310 15 b. I think this panel should be fed with #1 al. (dont have the book right here) I beleive that even if the water heater was fed from the main 200 amp panel, with that kind of load you would still see the lights dim.
