To remove dark water stains from hardwood floors, you will first wipe away any grime and dirt on the surface using a damp cloth. Secondly, soak a fresh rag in vinegar and then place it on the stain. Thirdly, allow the vinegar to lie on the stain for five to ten minutes. After that, test a hydrogen peroxide application in a subtle area of the floor. Read more on http://www.ehow.com/how_4968149_remove-water-stains-hardwood-floors.html.