This is the official line from the SW website:
Q: Will drinking plenty of water improve my weight loss?
A: Contrary to popular belief, drinking water doesn’t magically speed up weight loss or increase your metabolic rate. Feelings of thirst, however, are often mistaken for feelings of hunger so drinking a glass of water rather than a packet of crisps can allay that feeling (of course, with Food Optimising, you can reach for some Free Food or count Syns without worrying about adding inches!). Water also keeps you hydrated and lubricated, so it’s important to include around 6-8 glasses or 1½-2 litres of fluids a day (that includes low calorie squash, juices, tea and coffee.)