I had a four shop dry cleaning bus in london for 25 years.you can slightly fade water marks,but unless they came to us still wet,youve no chance of getting it out.if they are washable curtains you have a chance also if your not charging much for them dont bother!!
just another piece of advice,if anyone does shrink curtains,some (but not many) dry cleaners may have a sandershade stretching machine that can work in a lot of cases.it basically steams under tension
i had one it works only 75% of the time good luck
gary bristowclean