Water Marks (or water spots) are caused from hard water evaporating on the surface. The water carries minerals and contamination, and as the water evaporates off, these contaminants are super heated and left behind on the surface.
Optimum MDR will work best on a surface that has been coated and protected with a product such as Optimum Gloss-Coat where the water spots haven't had the chance to actually etch into the paint.
Optimum MDR is non-abrasive, and is applied by hand with a foam or microfibre applicator. Gently massage the gel into the water spots for 30-60 seconds will the MDR slowly dissolves the minerals. Rinse clean and check the surface for any remaining marks.