For places where bathing water quality measurements (to measure the suitability of water for swimming, not drinking) are taken, there is the tag ref:EU:bwid in the EU, and for a location outside the EU I’ve started using designation=bathing_water with url:bathing_water to link to the external web page where water quality measurements can be found. The water quality will vary over time, so linking to an external database could be more stable.
For a physical device that measures water quality, there is man_made=monitoring_station monitoring:water_quality=yes.