You can always specify tap water when asking in restaurants and that should be free. Its what we always do. If you just ask for 'water' you automatically get the bottled stuff.

The wine that gets discarded is probably drunk by the staff after closing. Most restaurants will not charge for rejected wine (its considered a write off) as usually the main reason for rejecting it is that the bottle is off and that means they can pass the complaint up to the supplier and recover their money that way. Even if they don't, the massive surcharge they put on every bottle is enough to cover the occasional rejected bottle and it is better from a customer service PoV to take the financial hit for one bottle of wine than incur worse from bad publicity from a complaining customer when you have charged them for a bottle they did not drink.