I ran a harmonised water quality lab on behalf of the EU.
"Water condition" covers chemical, physical and micro-organisms. The number of tests to get a reasonably good picture ran into hundreds, and cost a fortune to do.
Bacteria (E-Coli and faecal coliforms) were always counted on cultured plates. Viruses weren't of great interest then. The bacteria gave a good indication of waste water treatment works performance.
If I had to pick just one parameter for water quality, it would be the clarity or turbidity of the water.
Almost always, a high turbidity indicated a waste water works not operating correctly.
Turbidity is a robust measurement that can run unattended for weeks or months.
If additional measurements were needed, they would include dissolved oxygen level, temperature, pH and conductivity.
Even 30 to 40 years ago, dial-up modems were used to interrogate the data, but still required regular physical checks and calibrations.