This site has the explanation in French. Essentially it refers to something that is very clear and obvious.
http://www.linternaute.com/expression/langue-francaise/202/clair-comme-de-l-eau-de-roche/
what means "clair comme de l'eau de roche???"
4 answers
exact translation
clearly like water of rock???"
clearly like water of rock???"
clearly like water of rock???"
More like "clear as spring water." A literal translation will not help you here!
Mme
Mme