PowerMaids

Try these techniques:

Depending on how bad they are a mixture of white vinegar and water (50:50) is an effective way to remove water spots.

If they are very bad then you should buy an acidic based cleaner to remove. Vinegar is mildly acidic so it will remove only so much. Go to your local janitorial supply store and buy acidic based bathroom cleaner and tell them what you are cleaning. You should have good success.

Be sure to wear gloves and follow safety instructions on the bottle for use and handling.

Be sure to start from the bottom of the doors as well to prevent streaks from forming from the acid wash.

If you live in an area where you have hard water then hang and use a small plastic squeegee in your shower and give your shower-door and quick wipe down at the end of each shower.