Hi there,
It depends where you will be staying, because in some cities you can drink the tap water and in some others you can't ... and i think in the cities by the coast the water is not good for example in Sousse the water from tap is not good and there you have to buy the mineral water, and it's cheap it's not expensive at all ( 0.25 Euro cent for a bottle) in other cities like Kairouan, Kasrin etcc the water is good there and i believe it's as good as the mineral water
Cheers
Makrem