How can I clean water marks off old shower doors?

https://www.youtube.com/watch?v=pWe9bgKulK4

I use Mr Clean Magic Erasers you wet them and they work great then rinse once you are done

Clean them with vinegar, baking soda, and salt. Stubborn mineral buildup on glass shower doors is no competition for a few common household ingredients—white vinegar, baking soda, and salt. Spray vinegar on the door and let it sit for a few minutes. Next, create a paste with equal amounts of baking soda and salt.

Fill a spray bottle with a 50/50 solution of Dawn dish soap and vinegar. Spray it on the hard water stains and let it work. If they are tougher stains, you may have to scrub with a plastic scrubbie!

CLR works great too!

Scroll down to the bottom of this post for directions;

https://theforkedspoon.com/how-to-remove-hard-water-stains-from-glass/

MaryEllen: Lime Away works quite well too

Cut a grapefruit in half, cover with kosher salt and use as a scrubber.

Lysol Power Scum cleaner in the blue trigger spray bottle with a Magic Eraser or a scrub brush.

Wet & forget shower for a slower, no scrub solution. You can use 2x a week to accelerate the cleaning process then once a week to keep it clean.

I'd try an magic eraser before you resort to CLR or anything harsh like that.

https://www.youtube.com/watch?v=pWe9bgKulK4