Depending of the city/region tap water might be great or might be rubbish. Even Sofia different suburbs have different water quality and taste.The best one will be if supply from Rila mountain spring water, it's great taste and cold temperatures during summer. In general the tap water is fine and can be drunk, the one from towns anyway.I and my family do not drink tap water at Black sea resorts, new buildings, cutting corners, doggy construction, doggy water supply etc. I do recommend drinking bottle water(Devin is my personal choice) when at Black sea resort.