That's why I don't try dent repair unless,the piece is unfinished and even then I don't usually. If a compressed wood dent, the steam and heat could raise a bump that you then need to sand down. If it is an old piece it could be shellac. If not then likely a varnish of some kind or lacquer. Test in an inconspicuous place as @duncuss said with some denatured alcohol or even Everclear. Once you establish what finish it is, you can choose what watermark repair process. Sometimes you just have to get the stripper and sander out and refinish