Water marks can mar the surface of a beautiful piece of marble. Luckily, these are not true stains, but just mild damage caused by a reaction to something in the water, either an acid or a hard mineral. Removing the water mark is as simple as buffing it with a marble stone abrasive.
Pour about a tablespoon of marble etch remover over the water stain. These removers are available in the local hardware store and are used to polish marble stone.
Use a clean rag to gently rub the etch remover into the water stain. Rub the water mark for approximately 3 minutes to buff away most water stains. Wipe up the etch remover.
Repeat the buffing step two or three more times with etch remover at 3-minute intervals each time for deeper water stains. If the etch remover dries out while working, add a little water to it.
Remove very light water stains or finish up buffing with a fine grade steel wool pad.