Mix up a paste of baking soda and water to the consistency of tooth paste. With a rag rub a small amount of the mixture onto the area of the water spot in a circular motion.
For what it's worth this also works well on stains on a corian top stove.
For more information:
Read "I have marble bathroom tiles that are quite heavily soiled with soap scum. Can you recommend a nontoxic but effective cleaner/sealer?" a Q&A answered by Molly McCabe.