You must water Rhododendrons, Azaleas and Camellias with rainwater.
I’ve heard this many times in hard water areas, based no doubt on the fact hard water contains lime and lime is used to reduce the acidity of soils and composts. While it is true Rhododendrons and other ericaceous need acidic growing conditions the amount of lime in the hardest drinking water, such as my local one, is not going to be sufficient to effect the plant you are growing. Nurseries and garden centres large and small happily grow rhododendrons etc. and I have never come across one that didn’t use the ordinary tap water regardless of how hard or soft it is.