This is the official advice 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. 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.)