Provided the water is cool, is applied sparingly, 
and is removed quickly, there shouldn't be any 
effect on the paints. I use water to remove 
extremely stuck sketches from time to time and 
have seen no damage to the paint. However! You 
should ONLY use distilled water when water will 
be coming in contact with a cel. (Also make sure 
to check the ppm - or "parts per million" - of 
trace substances so you get the one with the 
smallest amout. Not all distilled water is created
equal.) Even filtered or spring water has too much
"other stuff" (like trace minerals) which can 
leave a residue on cels.

However, washing the cel with water will not 
remove the "dusty" look to which you are 
referring. I have an extremely "dusty" Cutey 
Honey cel that has seen every cleaning attempt I 
have heard of in an effort to remove the "dust". 
Nothing has worked. (The only thing I have not 
yet tried are mineral spirits, but only because I 
haven't gotten around to buying any.)

If you do need it or want to get some, distilled 
water can be purchased at most decent sized 
pharmacies and at some large supermarkets.

Many Sharp Smiles,
--Drac