When you boil dirty water, how clean would it be? Is it drinkable?
How many percentage does the bacteria in the water die if it is boiled?
100%. More info here: How long does water need to be boiled for to kill all bacteria / viruses?
How clean is it after boiling? Is it drinkable?
It will still have any dirt, etc., but that isn't necessarily bad for you. Boiling will not get rid of chemical contaminants such as selenium or toxins that can be produced by algae. If the water is really nasty and murky, then if only for the sake of taste and aesthetics, you may want to filter it through a bandana or a shirt first.