I had the same issue with my first batch. I was about 3/4 of a gallon short and wanted to add water also. Everybody said not to do it because it could contaminate the batch so I just went with it. My OG was way of and my finished gravity was moderately high. Other than that it turned out to be a really good beer and I had a few less bottles than I had originally anticipated. Roll with it and I'm sure it will be fine.