I was always confused by the phrase “water weight” because I didn’t understand what it meant. It’s glycogen - basically a way our livers store energy with 3 molecules of H2O with every carb. So after you burn through the energy stored as blood sugar, you use up those carbs and shed that water with them. This is why initial weight loss can be so much the first week, but then burning fat is only a pound or 2 a week.
TBH I still don’t totally understand it (probably obviously) but it helped me realize that water weight isn’t just what my bladder can hold plus blood volume. Glycogen is your search term for understanding it better than I do!
Edited - oops! 3 water to 1 carb