Hello and welcome, Wyldchyld.
Congratulations on the loss, Reluctant, and on your walking, Sleepingbunnies. moonshine, fantastic that you've hit a milestone, and welcome to the 15s!
Mary, I'm afraid I have no idea of the science about water loss. I just know it happens. Well, it happens to me. The more I eat (especially carbs), the more water I seem to retain. If I eat less than usual, I lose water. I know this because of what the scales say, and the amount of times I have to go to the loo, especially during the night!
To give an example, I've just started 5:2 and I'm weighing daily so that I know how my body is responding. (I don't usually weigh daily, just when I'm changing to a new pattern.) Saturday was a fast day. On Sunday morning the scales showed that I'd lost 2 lb since Saturday morning. On Monday morning (having eaten about TDEE on Sunday, so you would expect no gains or losses) I was a pound heavier. But the 2lb loss on Saturday wasn't all 'real' loss - a pound is about 3500 calories so it couldn't be. Most of it was water. And the pound gained on Sunday was some of the water being retained to deal with the food I'd eaten, or however it works. And so on.
I'm afraid I've no idea why this happens, or the science behind it. I just know that if I start eating less than usual I have a sudden spike in weight loss which tails off, and I have to go to the loo two or three times in the night.