dandruff is a fungal infection of your scalp hair.Oh yea drinking water is good but it has no relationship with dandruff.First thing is prevention,don't share towels combs,caps/hats.Oil and greasy applications favour fungal proliferation so do not use it.Frequent washing on alternate or daily is suggested.
For shampoos use shampoo that contains anti-fungal preparations like ketoconazole,selenium sulfide,pyritione zinc.The one you are using-Nizoral is a ketoconazole preparation i guess.if it hasn't worked you can try shampoos containing selenium sulfide ( Selsun ) .If still no pregress you might consult doctor and he will provide high-strength preprations.