Typically with any stain, the faster you send it to wash - the easier it is to get rid of it. If it's just a cushion cover, dumping it into the washing machine would save you heaps of time.

However, if you aren't planning on washing the whole cover anytime soon, you can definitely try a mixture of white vinegar and water (equal parts), spray this around the affected area of the water ring, and blot it with a dry cloth.

Alternatively you can try lemon juice and baking soda; apply the baking soda over affected area followed by a squeeze of some lemon juice. The concoction should foam like soap and you should be able to clean it with a slightly damp cloth.

These are quick and easy fixes so having said that you'd definitely wanna test it out on some place inconspicuous first though to make sure it doesn't do anything else apart from leaving your sofa fresh and clean!