Water should not remove paint, but prolonged 
exposure to water can warp the cel.  (think old 
transparencies that you've seen in school)  
Obviously, you don't want to drench the cel.  I 
just dip some paper towel in water, drain off the 
excess, and wet the paper.  After a little 
waiting, *slowly* peel the paper off the cel.  If 
the paper does not want to come off, just add a 
little more water.  After removing the paper, I 
dry off the back off the cel with a photo cloth.  
I've done this on two occasions so far without 
any problems.