I spent most of December in the hospital after back surgery. I also have some cardiac issues and the cardiac docs put me on intensive diuretic therapy. That takes the water that has accumulated in your legs and abdomen/chest and moves it into your bladder where you can just pee it out. I lost 46 pounds that month and it was all water weight.
My experience was probably extreme, but I had always assumed it was fat. I was shocked beyond belief that it was all water and I could just get rid of it by taking a drug.