Grab a damp cloth and apply a little dish detergent to wipe over the stain in a circular motion until clean.
(It’s worth being aware that water stains on marble tend to be caused by hard water only.)
how to remove stains from marble with baking soda (great for oil-based stains)
Wipe over the stain with a soft damp cloth (microfiber cloths are great)
Mix 1/4 cup of baking soda with a tablespoon of 3% hydrogen peroxide (note: only use hydrogen peroxide on white marble!)
Add warm water to make a paste.
Spread this paste over the affected area to dry overnight. (you can also cover it with plastic wrap)
In the morning, scrape the paste off with a plastic or wooden scraper (never anything metal, as this could scratch).
Wipe clean with soap and water, then dry thoroughly
Repeat again if necessary
how to remove stains without baking soda
Wipe the area clean with a damp, soft cloth.
Soak a paper towel with a 3% hydrogen peroxide solution to cover the stain.
Place a weight, such as a glass mason jar, on the towel to weigh it down overnight.
In the morning, remove the towel, wipe the area clean with a damp cloth, and pat it dry.
Repeat if necessary.
Notes
Safety note: Never use hydrogen peroxide on dark marble, as it can lighten the color significantly. Only use on white marble. Always test a small, inconspicuous area first before attempting either of these recipes.