For the cell shading, I'm mostly just practicing at it, which is working pretty well for me so far. It's a completely different way of going about finishing a piece of art, so most of my issues thus far are about how to implement it and handling background shading, especially for objects with a lot of gradual contours, but I'm getting my way around them pretty well as I go, so I don't think I need resources and such for that.
With water and rocks, I have taken some measures to observe them more through my time outside and through photo references, which helps. I also look at how other artists handle them, which can help a lot with actual application. I probably would be improving with water faster if I practiced it more, though I have a lot of art ideas to do and as of the moment, not very many of them include water. The observation has helped me with drawing sea water, since whenever I go to the beach I take a lot of visual notes. Going to streams and rivers would be a great idea for other forms of water, but I would have to wait until I actually have a chance to go to some of those.