No tap water in Guatemala should be considered potable. Municipal water supplies vary wildly in quality from city to city and town to town. There is nowhere in Guatemala where the running water should be considered safe to drink. All drinking water should be purchased from stores.
In some areas running water is simply pumped from nearby natural sources (rivers, lakes) into holding tanks to supply homes or commercial buildings. This water is not safe to drink, and some people may not even want to brush their teeth with it. Sensitivity to parasites and amoebas vary.
Buy all drinking water. Small 20 fl oz / 600 ml bottles of water cost about Q3.5 - Q5 ($0.46 - $0.65), gallon / 3.78 liter jugs around Q10 ($1.35) and large five-gallon / 18.9 liter containers (pictured) cost about Q17 - Q20 ($2.26 - $2.65). In Guatemala drinking water is called agua pura (pure water), not bottled water or drinking water. It's smart to buy water in the evening so that you aren't without it in the morning, waiting for a shop to open.
Many hostels and hotels have free drinking water, but not all.