Water is a critical part of any weight loss plan (WW or otherwise). You need to flush your system daily. Water helps aid digestion, clean out your body, and gets rid of water weight (most people do not drink enough water on a daily basis, thus your body holds on to "extra" water for those times when you do not give it enough). But by drinking enough water, your body will let of its reserves of water...and thus you will lose more (particularly in the first two weeks). After that, though, your water becomes important not only for keeping you properly hydrated, but also for fat burning. The chemical compound of H2O actually breaks fat down.
You will want to drink a minimum of 64 ounces of water a day. If you are not used to this, it feels like A LOT, I will warn you now.
Other people do the 1/2 trick. This means you take your weight, cut it in half, and then drink that many ounces of water each day. For example, if you way 200 pounds, then you should drink 100 ounces of water each day. Yes, it is a lot, but it really does work.
Water is essential to overall health.
Hope this helps!