I have had situations where I will hit a new scale low and then for a week or so, I'll be back up to where I had been or even just slightly higher. However, I know I haven't gained fat, because I know I haven't eaten anything to lead to that. So, I remind myself that it's just water and keep on keeping on. I also know that on these days, I am getting ready for work and can't just stay home and drain off water to the toilet and then go weigh again. That's how I know that a lot of what I carry is water. I just don't count readings that don't account for all the water I carry and then drain off.
And, none of that takes more than a week. Every Saturday, by the afternoon, I'm down at least half a pound; usually much more.