What is water weight loss? How long does it go on for? Is it a bad or a good thing??? Does it mean I haven't been drinking enough? I had surgery on 8/8 and have lost over 8lbs since then which seems like a lot to me. I have been drinking a lot I think but not eating a ton - maybe 400-500 cals a day. Is this something I should worry about?

Nope, as long as you are keeping up protein and staying hydrated, no worries. Quick weight loss during liquids is common. Enjoy it while it lasts.