Besides the fact that you'd literally die without it, there are many, MANY imperative reasons to drink water frequently, every single day. It starts out pretty mild — you might feel thirsty and have a dry mouth. But the long-term effects of not drinking enough water not only have an effect on your weight (in a bad way), but they're also extremely dangerous and life-threatening. Here's what happens to your body.
Even mild dehydration has strong effects. Here's how you'll feel with a lack of H2O (hint: it's really not fun).
If things get worse, so do your symptoms. These are the "go to the hospital" signs.
Consistently not drinking enough water for an extended period of time has its effect as well. Although you may brush off the milder side effects, your body is still suffering — and several of these have a significant bearing on weight gain.
Now for the worst of it. Yes, it's terrible that dehydration can make you gain weight (or keep you from losing it), but there are some bigger issues at hand. If you're truly neglecting your water intake, this should likely help you get on track. Here's what happens to your body when you don't get enough water.
Now go get yourself a water bottle and FILL. IT. UP.