Best bet Mark is go get some Megs Quick detailer spray this on the damp surface then dry off and you should not get any dried water marks. Can get a bit expensive buying it in little bottles from halfrauds so i get a 5litres or so and you can dilute it.
Small bottles are around £6.99 large drum is about £18.
The other option is to get a water filter or actually pour still water like you get in water machines over the car. I have never gone down this route. But is meant to work really well.