The best product I've found for water marks is Koch Chemie FSE. It has a gentle limescale remover in it, which is perfect for removing water spots (it also acts as a great quick detailing spray as well). Spray it on, leave for about ten seconds and then buff off with a decent microfibre cloth. If the spots have been on for a while or are more ingrained into the clear coat you could try a decent polishing compound like Meguiars ultra finishing polish, either applied by hand or a dual action polisher, if you have one.