of course its going to have watermarks and dust on it such is life im afraid. the wax will make the water bead though and run off of it rather than sitting in puddles looks pretty
if you dont have grit guards just make sure you dont dip to the bottom of the bucket or kick the ater up to much and you'll be fine
let the wax sit for a while to cure before adding the detailer and sealing it in
you can indeed clay the windows, leaves a good finish which rain x repellant can be put on to bead the water off the glass