Nuclear, food products, and pharmaceutical often specify specific grades of elastomers and require cure date information for certain products. If you are working in any of these areas check for a list of approved materials. The term water does not describe a single product. The result is that sometimes the water can attack stainless steel and some seal face materials including carbon. You may have to do some immersion testing to be sure if your choices are satisfactory.
Water treatment varies with each application.
These treatment chemicals and additives can attack some elastomers Condensate often contains dissolved amines that could attack the elastomer. Water hardness varies with geographic locations. Waste water is liable to be any thing. If you have any doubt about your water, conduct an O-ring immersion test prior to installing the mechanical seal. Look around the plant for present or past experience.