Did a google on water weight. This is what I found:
Many average healthy adults experience some water retention and the accompanying weight gain that occurs. The day to day body weight fluctuations are commonly caused by the loss and gain in body fluid.
Its possible to retain up to five pounds of water weight retention and can easily be hidden within the natural fluid that surrounds cells ( extra-cellular fluid ). Heavier people may experience more water retention especially if their intake of processed, convenient food is high. For some it can be as much as eight to ten pounds of fluid retention if weight gain is quick say over the Christmas season.
There are a few causes of water retention but for many an increase in sodium through the diet ( mainly salt intake ) is a common cause and research suggests it may contribute to high blood pressure and other health complications. Sodium is present in virtually all food products so when a person consumes more food its inevitable they also consume more sodium. Weight gains are very often a combination of fat stores and water retention.
Water regain & weight loss
People who lose weight quickly usually experience some weight regain within a few weeks and very often this is due to some water replenishment.
When we restrict energy intake too much too soon in order to lose weight quickly the body is forced to use up stores of carbohydrates and breakdown protein in the muscles. As both carbohydrates and protein hold water in the cells a loss of these macronutrients also results in a net loss of water. As a result rapid weight loss can often be made up of 75% water loss. After the energy systems stabilize water is regained because some of the protein and carbohydrate stores initially lost are replenished. The water is drawn back into the cells thus gaining back a little weight.
It can be very confusing when one week you lose 4 pounds, the next week nothing. However remember that gaining protein in muscles increases lean body weight. More lean body weight increases our metabolism and help burn fat faster!
OK, found another really good website with great info on weight. It's called Why the Scale Lies (Hmmm, it doesn't show up until you put your cursor over it.)