I guess I always knew water weight is a significant part of body weight but had never really quantified it, just had a rough idea could account for up to 5 lbs after a hard workout . . . so the other day, after a hard 20 min workout on the bike trainer, went to weigh myself and, sure enough, my body weight was 4-5 lbs lighter than usual, which I like to see, but then was very thirsty so filled up my water bottle and before drinking, thought, "I wonder how much weight I will gain back if I drink this water" so took it to the scale, hopped on the scale and sure enough the scale read x+1.5 lbs where x was the body weight without the full bottle. then just to confirm, I got off the scale, drank the entire bottle, hopped back on, and it read x+1.5. yay, water in the bottle was now water in my body, and the scale read the same. Then I hopped off, set the empty bottle down, hopped back on the scale, and it read x+1.5 still . . . so evidently, the bottle itself did not weight enough to tick it up to x+1.6 lbs. anyway, maybe all this is obvious, but it's nice to know how water replenishment relates to body weight. and that my scale records the water weight accurately.
as an aside, I read somewhere that you should weigh yourself before strenuous exercise, then afterwards, drink fluid to bring your weight back to pre-workout levels. then you know you have replenished fluids. but I've lost as much as 5 lbs in a long bike ride, so that would be about 3 liters of water (4.5 lbs by the above measurement), which is a lot to drink.