Same here, I have very hard well water and no city water. I don't wet tumble but I do clean with citric acid. After the brass is dry I run it through the dry tumbler with corn cob and Nu-Finish to get the water spots off. Probably not a palatable solution to the guys that wet tumble.
A wetting agent like Jet-Dry might help or Kodak makes a wetting agent called Photo-Flo 200 that is used to keep film from water spotting while it dries after processing. The dilution rate is 1:200 so a little goes a long way.