It certainly depends on the diet. As James indicated, low carb diets tend to have a diuretic properly. Other diets that don't concentrate on low carbs will have less less affect on water retention.

I suspect that it is more to deal with exercise where water retention can be more easily manipulated. Lots of exercise and lost of water intake with low sodium means that all access water will be flushed from the system.

Zdiets.net - Lost 170lbs, No more Diabetes, No more Hypertension, No more Cholesterol Issues.

Yes and no, that part depends mostly on your salt/potassium balance and how many carbs you have running around your system.

But as you let go of fat, fat tissue seems to get less dense and in the place where there was a thick layer of fat, it hangs on to water, then at some tipping point it just lets it go, and you have this weird effect of having no weight loss at all for a few days, then overnight you drop 3-4 lbs. So using a scale for fat loss monitoring doesn't work too well.

see: http://www.bodyrecomposition.com/fat-loss/of-whooshes-and-squishy-fat.html