To keep water spots off my black truck i add a cap full of white vinegar to the soap mix......and dry it off when wet.....dont let it dry any by itself.
I also just got that MrClean car wash deal with the filter and soap built in......let me tell ya i am impressed!! I was sceptical about letting the truck dry on its own.....but it worked! And quite well i might add! I think i may use this on the boat once i have it done!