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

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?
Post Opinion