How do you remove water stains from toilet?

Use a pumice stone

Try a commercial toilet cleaner called ZEP. It cleans pretty much anything off of porcelain.

Make sure that pumice stone is for plumbing fixtures

I clean houses for a living. The best toilet bowl cleaner is Limeaway. It's $1 at Dollar Tree. Put it on and let it sit for and hour or two. Most of the lime scale buildup will come off with the brush. If there's more I use the Pumie on it. https://www.amazon.com/Pumie-Scouring-Stick-HDW-12T-4PK/dp/B00N3EI34U/ref=sr_1_2_a_it?ie=UTF8&qid=1491858052&sr=8-2&keywords=pumie

I like Ka Boom You can get this Ace -Home Depot or Lowes

I like the Lysol toilet bowl cleaner in the BLACK bottle. It worked wonders on the hard water ring in my toilet.

Drain the bowl , and use fine sand paper. It will not scratch the surface.