If they are the stock aluminum wheels, they will have a clear coat and should ploish up with rubbing compound. This will remove hard water stains and also shine them.
If the white areas don't come out, it's possible that your clearcoat is separating from the aluminum (very common). The only solution to this that I know, would be media blast the clearcoat off and reclear them.
It may be more practical to replace them.
my
-slacker