Kathy Love will tell you that if you use tap water to leave it in an open container for 24 hours before giving it to your snake, as then the chlorine evaporates off and yes the chlorine can be harmful to your snake, esp when little hatch-lings. Fluoride can also be harmful. If you do not want to use tap water, spring water is the best choice because it has naturally occurring minerals that they would get in the wild and do need but distilled and filtered water removes those minerals.
All mine have tap water, but I usually add the proviso that if I wasn't happy to drink it, then I wouldn't give it to my snakes.
Tap water can contain chemicals though - residues from old pipework and such, as well as occasionally concentrations of bacteria that means water has to be boiled before drinking. These are man-made hazards and not the sort of things you'd find in nature at all.but snakes drink water out of dirty stumps, streams filled with bacteria, stagnant ponds, and puddles on the forest floor.