Water retention is definitely a cause of extra weight. We are used to/trained to not drink as much water as our bodies actually need. Fixing that by drinking a lot of water is a quick way to lose some pounds. Of course it doesn't work if you're already properly hydrated and it only works until the problem is remedied; you will hit a plateau early in your diet. And it doesn't work if your consuming more calories than your burning. But if you're already dieting (which the grandparent was), it's absolutely a good way to kick start your diet.
