If it's a white water ring, the first thing you can try is using a hair dryer to see if that pulls the moisture out of the wood. If it's a dark (or black) water ring, it can be more difficult. I did a blog post on how to remove both white and dark water stains from furniture to give you some solutions: https://www.simplicityinthesouth.com/how-to-remove-water-stains-from-wood-furniture/
Try the product REstore -a - Finish. It will remove white rings and scratches.
You can also try Tung oil. Tung oil will brighten up the wood and is good at getting water rings out. Best of luck to you!
Here's a useful method
Craft Invaders has got it. Cloth over the stain and an iron set to medium heat.