| Those marks on your roof do sound very much like water marks. To get rid of them get some meguairs body scrub, zymol hd cleanse or Swissol Paint cleaner (meguiars is £6.99, zymol and swissol are both around the £20 mark). Follow the instructions on the bottle and then apply some good quality wax.
If you used a jet washer at a petrol station etc they use a very strong detergant to clean of all the road grime, brake dust etc. The down side of this is that it also strips the wax/polish of your cars paintwork. This will then cause water marks in your paintwork, you'll more than likely find they are all over the car, just not as visible as they are on the black though.
J. | 
