Nothing is Avoda Zara unless you worship it. Doing something that someone told you or you thought would help, for whatever reason, is not Avoda Zara.
Nichush means doing something based on a superstition, not doing something to get results.
Darkei Emori only applies if it is in fact a superstition that originated by Goyim. The Chida says that we can’t add anything to the list of Darkei Emori that Chazal gave.
So, hitting a stone to get water would be, at worst, Megale Panim Batora Shelo Kahalacha, since you are showing that you think that the water was not a Ness.