I have well water and we have a lot of trouble with rust and lime residue in the tub and shower, not to mention soap scum. The product I use to clean it with is called The Works Tub & Shower Cleaner. It is fantastic for cutting through the grime and making the tub and shower shine!
I have used a lot of different type of cleaners to get my tub and shower clean, but I have never had another product do as well as this one. I won't even buy another brand anymore! All I do is squirt it on the wall of the shower and the bottom of the tub and wipe with a towel or sponge. It usually takes the mild dirt off immediately, but the tougher stuff might take a second application. I never have to scrub hard, the solution does all the work, well almost, I do have to do the cleaning, I just don't have to work as hard at it.
See my review on The Works Toilet Bowl Cleaner, it works just as well on the toilet bowl.