There has been a lot of progress in the dissemination of data about French water quality since 2016.
All the raw measurements of the concentration of substances in rivers and lakes are now available on http://www.naiades.eaufrance.fr. This accounts for about 1000 different substances observed on a network of 5000 stations with a frequency of 6 to 12 times a year. The oldest data goes back to 1970 and the newest is about 2 years old, which is the time needed to qualify and validate it and collect it at the national scale.
The same data is also available for groundwater.
Up-to-date data about drinkwater quality is available on http://solidarites-sante.gouv.fr/sante-et-environnement/eaux/article/qualite-de-l-eau-potable, for each of the 36000 municipalities. This data can not be downloaded at once though.