How to remove water stains in toilet?

Lysol toilet bowl cleaner in the black squirt bottle is great. Coupled with Mr. Clean Magic Eraser will work wonders. I had a toilet with serious stains that bleach wouldn't budge but this combination worked without damaging the finish.

Calcium Lime Rust remover, multiple applications, hand scrubbed with a nylon scrubber.

I use a pumi stick for stubborn rings or stains in the toilet.

Muriatic acid will probably remove it, but precautions need to be taken. You need to have a fan that will blow air across the toilet and out a window. A mask is necessary, but I found a thick folded towel over the nose and mouth works better. It puts out a gas that is toxic to the lungs, but I found the towel does much better. I have lived many places with hard water and my son likes to ignore cleaning his bathroom toilet and it cleans it very well and quickly. I remove the water from the toilet bowl and drizzle it over the inner bowl, let sit for a few, then use a toilet brush to scrub and bring more of the solution up to the area you are scrubbing. Turn the water back on and flush the toilet a number of times to rinse it and the toilet brush out really well. If you have birds, or any pets for that matter, make sure they are in another room far away from the bathroom so they do not get any stray gas inhaled.

COCA-COLA. Just pour into the toilet & let soak overnight. It will scrub out clean the next morning. Flush. If it doesn't remove all of the stain, repeat & it should work. It's also great for doing burnt on or crusted on pots & pans. It will even take rust off of cars & tools. The acid inside the Coca-Cola eats the stain & rust.