If dehydration had any noticeable effect on weight loss I'd still weigh 190 pounds. I am always slightly dehydrated; getting my water in is my single biggest challenge health-wise, heh.
OK that said -- yes, water weight does that. As has been said, you may notice it less as you lose weight, or if you get your intake of things like salt, sugar, alcohol (and of course water) to a very consistent and steady level. And you'll want to weigh yourself under the same conditions every time, first thing in the morning being the most reliable. (And then never pay attention to weights later in the day, because you *will* "gain".) But it's still going to happen.
Since the amount of weight that can be gained or lost purely from water on a day to day basis can equal or exceed the amount of real weight you might expect to lose in 2-3 weeks, this can easily be very frustrating to people. You have a couple of options for keeping your sanity. First, you can just not weigh yourself very often. The most reliable method (for a woman) would probably be to weigh only once a *month* -- I've seen suggested to do it 7 days after the start of your period each time. This means there won't be any effect from hormones messing things up.
The other way, if that's just too long, is to weigh more often, even once per day, but ignore the minor ups and downs and look at the longer trends. Over time you should be trending down. As long as that keeps happening and you keep seeing the new lows every so often, there's nothing to worry about.
Height 5'8 1/2"
GW: Originally 150. Maintaining at about 146 since June 2013.
5K 4/21/11: 31:55
|39 Maintenance Weeks