Water weight is one of those mysteries of dieting.
Here's the thing: Your body is made up of something like 70% water. Your cells - all of them - are filled with water. Your blood is mostly water. Your brain and spinal fluid are mostly water. Your lungs are full of moisture. Bile, digestive fluids, saliva, mucus ... all water.
So with that much of your body being made of water, an "imbalance" one way or another has a huge effect on what the scale says.
If you don't consume enough water, your body holds on to what water you give it - and you retain.
If you consume too much salt or sodium it causes an imbalance in the ratio of water to electrolytes - and you retain.
If you workout and start to sweat a lot - your body might show a water loss until it restores it's natural balance.
If you start working out and really work your muscles hard - they'll retain fluid (mostly water) to aid in the recovery and healing process. It's very normal for someone to "gain" 3-5 lbs when they first start lifting weights and that's where the myth that you've "gained muscle" comes from - but the truth is that it's just fluid and you don't build muscle that fast.
If you start a diet that's very low in sodium - your body will dump water until it achieves a natural balance.
If you change the amount of carbs you eat (or the types of carbs), it can affect how your body retains water
If you're menstruating or ovulating - your body might retain water or your body might shed water, it just depends on your particular biology. Most women retain, but some do actually drop pounds over their periods.
Humidity outside - believe it or not you lose moisture when you breathe. If you move from one environment to another you can lose more water by breathing in a dry environment or retain water by being in a humid environment.
Are you ill, or fighting off an infection or a cold? Your body will retain fluids to help with that process to make sure you don't become dehydrated.
There are so many things that can influence water weight. That's why it's SO important to not be emotionally tied to the number on the scale. It's a good guideline ... but it's not gospel.