Do ceramic coatings prevent water marks?

Ceramic coatings don't prevent water spots because these are caused by minerals and other contamination in the water. When the water dries, it leaves behind these deposits, causing rings. Because ceramic coatings are hydrophobic, there is less water on your car, so there will be fewer watermarks. They are also easier to get off of a car which is properly maintained. 

In this area of the country, we don't suffer especially from pollution, so water spots are likely to come from the tap water in which you wash your car. This can be reduced with inline carbon filters, drying agents and good drying towel skills.