The water is just the carrier to get the Ferric nitrate into the wood. As Chuck said use distilled water. I make my solution 5 or 6 to1 as I can always reapply. I do this because every piece of wood reacts differently. I can always darken it by reapplying, but it's hard to lighten a piece. I have reapplied as many as three times. You really need some scrap, or a hidden place to find out what the A/F does to that particular piece of wood.. Just because it's a piece of Maple doesn't mean it will stain the same.
Bill