It depend how serious ur watermarks is. basically there is 2 types of watermarks on car paintworks.
1st type will be watermarks on paintwork, second type would be water etchings which gone into the clear coat.
1st type you can remove by wax or polish. Which product will depend what expectation or any concern on branding. First thing you can do is clay your car. after claying you can do some paint cleaning. If you want a cheap solution i would recommend soft 99 cleaner wax which cost rm17-22. If you want meguiars' product i would recommend ColorX (polish + wax).
If your watermarks is second type then you have to send for machine polish.
Bellow is a link from meguiars online hope can help you understand about watermarks.
http://www.meguiarsonline.com/forums/showthread.php?t=4891
Some tips to prevent watermarks for you.
1)Always dry your car after washing! this is the best way to prevent watermarks.
2)not washing under direct sunlight.
3)Keep the paintwork wet during car wash until drying process!
Hope this can help you.