I don't think you can retain water simply by drinking a lot--the healthy body naturally will flush out excess.
The water weight that people lose is typical on low carb diets in the first week. Apparently, there are glycogen stores in the muscles that are dumped due to lack of carbs (moving from burning carbs for energy to burning fat). That's why when people go off plan and eat a high carb meal or two, they will report "gaining" 3-4 lbs 'instantly.' It's the water retention--reversing the process that low carbing causes.
In addition, the body doesn't like losing weight. So at various times in the weight loss process, people will worry about a "stall" of a week or two. What can be happening is that as a person empties fat cells (burned for energy), the body will fill those same cells with water. This is always temporary, but the individual who relies on the scale will assume that he/she is not losing (or even gaining a little), when it's just the body's last ditch effort to hold onto weight.
Also, individuals often have unique water retention issues. For example, I'm hypothyroid, and if my meds are off, I'll find myself retaining water. In addition, throughout my life, if I get a virus or the flu, for some reason my body will instantly retain a lot of water. As I get better, I find myself in the bathroom a lot!