You can rub it with a teensy tiny bit of denatured alcohol or rubbing alcohol. Yes, this cuts through the finish and yes, it’s going to be a little duller. But it allows the moisture to escape and thus the ring will be gone. Don’t rub so hard you actually hit bare wood. It’ll be fine. ^_^

I used to have a water stain on one of my finished tables, but it seems to have disappeared over time.

I sometimes rubbed parts of that table with slightly diluted vinegar just for spot cleaning: that might have been a factor in removing the stain.

You could try rubbing a small part of the stain with diluted vinegar and waiting for about 24 hours, to see if that helps.

I have not tried this and don’t know if this works or why. But someone told me mayonnaise put on in over night treatments.

I have had success rubbing the area with tooth paste, not the gel formula though. Then if the area looks a little dull rub it with a walnut half. Walnut halves work great on scuffs and scratches too in wood.

So thanks all. I tried the mayonnaise, which has worked for me on floors, but didn’t do much here, unless it prepared the stain. Then I put on some toothpaste with a bit of baking soda, and voila! it rubbed out. I should stay, the mark was light almost white, which I think meant it wasn’t too deep. Anyway, this had been troubling my Dad, so I was thrilled to be able to solve the problem for him.
Yay!