Water marks or white water rings are usually due to moisture getting trapped in the coating.
Use a hairdryer to try to draw out the moisture. It is important to have the hair dryer on a low heat setting and keep the hairdryer moving. As the coating gets warm, the white marks should start to disappear as the moisture evaporates.
Alternately and provided the surface is fully cured, you could lightly wipe the area with a clean rag or cotton wool saturated with methylated spirits. The methylated spirits may draw out the moisture from the coating. The cloth must only be damp, and you must wipe gently, otherwise you run the risk of damaging the coating.
Water stains are likely to dry out on their own with time.
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