If you put an excess amount of water into your mix you will lower the specific gravity (which reduces the potential alcohol of your wine) and it will also dilute the body characteristics (your wine will be less full-bodied).
If your intent is to make a more full-bodied wine and you intentionally underfill your fermenter there are a whole different set of potential consequences. Wine kits are manufactured with a certain quantity of wine (30 bottles normally) being the intent. Acid levels in the wine are balanced with this in mind. Underfilling your primary fermenter may cause your finished product to be overly acidic. There are other possible consequences as well including the disruption of the typical fermentation process.
BlueStem Winery offers a complete line of winemaking ingredients and also is well-stocked with home brewing ingredients, equipment and supplies. We put together our own line of home brewing ingredient kits known as BlueStem's Best plus we are proud to be retailers for both Cellar Craft kits and WinExpert wine ingredient kits. Our website features a handling fee of $6 per order with all shipping expense included in the price of each item purchased.
For prompt, knowledgeable and friendly service BlueStem Winery can be counted on to be there for you!