Can you gain water weight from soup? If so, how long does it last?

Anonymous
My work is doing this weight loss competition. I decided to join because I wanted to lose ten pounds before the summer, and I figured this would be the best motivation. I wouldn't say I'm exactly fat, but I've never felt comfortable wearing a bikini. I want to this year because I saw a really cute one at the store that I want. So I've been eating healthy and exercising and I ended up actually losing more than what I set out to lose. Tomorrow is our weigh in. So I decided that just for today to only eat soup and skip any solid food. One of my friends said that is one of the worst things I can do because it's going to make me gain water weight.

Is this true? If it is, will the water weight be gone by tomorrow? I don't plan to eat anything after 8 tonight anyway.

Overall I don't care about water weight for the long term because it will be gone, I just don't want to screw my team members over. They didn't want me on their team originally because they didn't think I was fat and so they didn\t think I could lose weight. Also tomorrow is the mid way weigh in, so there is another weigh in in May and on Friday I have surgery, so I will lose weight just from being in the hospital.
Updates
+1 y
It went well. I was actually 3 pounds lighter than I thought I was. So now I'm back to eating the normal yummy food. I don't think I am going to do that again for the final weigh in though. I'll just stick to what I've been doing because it is working. Eating nothing but soup for a day wasn't easy. One girl another team is doing that everyday and I don't know how she can do it... especially since she'll likely gain it all back anyway.
Can you gain water weight from soup? If so, how long does it last?
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