Take your pick:
Mix 50/50 toothpaste and baking soda, rub in. Do not use gel toothpaste for this.
Rub in a paste of salt and olive oil, allow to sit for up to an hour. Wipe off.
Rub in Miracle Whip (Mayonnaise) and wipe off after an hour.
Mix 50/50 vinegar and olive oil and rub into the watermark.
Try straight toothpaste (non-gel). Rub in with the grain then wipe off.
Make a paste with baking soda and a few drops of water. Rub in then wipe off. You can also try salt instead of baking soda.
Rub some Vaseline (or other petroleum jelly) into the watermark and leave overnight. Wipe off in the morning.
Or, our all time favorite... a simple steam clothes iron, a clean white dish towel (dry)... set the towel on the mark, use medium high heat on iron and move in slow circular motion on towel after steam has started coming out of iron. You may have to do this two or three times to be effective...