>go explain that to people every time you are drinking water in a pub

Why do you feel a need to explain it? I've never had any reaction at all with 'no thanks, I'm good'. You don't have to say 'I'm on a diet' 'I don't drink' 'that crap will kill you' or anything else. Just no thanks. If someone does ask about your water, say 'I'm taking it easy' or 'I'm good'. Or get a club soda and lime.