you'll only get sick of drinking the tap water here if you've got a really sensitive belly. if you drink a few sips here and there nothing will happen. still I wouldn't recommend it, the taste is awful and depending on where on the island you drink tap water from, it's really not good for your health in the long haul.
I often drink a beer every now and then (not too much alcohol though) and I always have a bottle of water in my pocket. if I do a full night of clubbing I'd say I drink at least 3-4 bottles of water if not more. especially in july and august when it's really hot it's just too important to keep your body hydrated, no matter the price. if you can't afford the prices of water in the clubs don't come to ibiza (but I of course agree that the prices of water are silly expensive in almost all the clubs).