Enter Blue Dawn!
I bought a $2 spray bottle at the grocery and filled it part way with vinegar. Keep track of how much you put in.
Spray on your bathtub, tiles, or shower door. Give it a few seconds. Then rinse well. I did use a sponge to help rinse so I did a little scrubbing too. Holy cow does this clean well! And the most expensive part was the bottle!! Enjoy!