Water is not your friend when it comes to wine tastings. I still remember the time at one of my favorite wineries in North Georgia, Wolf Mountain, when the attendant refused to allow me to rinse my glass with water. I almost felt scolded but he taught me a valuable lesson. After we finished the whites he poured a few drops of the next wine, a red, for us to rinse with. This was some years ago and the practice does not seem to have caught on, at least not here in North Carolina. Maybe it has to do with the perceived lose of revenue or just plain not knowing better. I was glad when I saw this article today from another blogger on the topic with the same opinion as I. You can check it out here: http://tablascreek.typepad.com/tablas/2012/11/when-wine-tasting-step-away-from-the-carafe.html
Water is rarely added at all during the production process, why would you want to add it during a tasting? Tasting samples are so small that a few drops of water can have huge impact on the flavor profile of any wine far more so that a bit of white wine in the glass.
I suggest you either rinse with a red or don't rinse at all and try to remove as much of the previous white wine as possible. I will sometimes run a napkin in the glass if I have one handy.