Beverley, I am assuming that you mean you have a white water mark or ring. If you left water on the table long enough for the mark to become black, you will have to refinish to remove it. The reason is that a black mark is under the finish while a white mark is in the finish.
Removing the white mark can be pretty easy in most cases. The first thing to try is to leave an oily material on the mark over night. The oil will work its way into the finish and replace the water that's making the hazy white mark. My favorite for this is petroleum jelly (Vaseline) but you can use a lot of similar things including mayonaisse. Even a little olive oil is good.
This will work in most cases and a second "over night" would be in order if the first night seems to be working. If it still wants to be there, you can take stronger measures but these will be ever so slightly more detrimental to the surface. The "secret" really is to use slightly stronger methods until a result is attained. Starting off with the strongest will work but will also show some evidence later.