No, excess fluid weight is not the same as excess fat. You can lose excess fat by reducing calorie intake and exercising more, so basically you are burning up more calories than you are taking in.
Fluid weight can be lost from the kidneys and by passing more urine, so yes, if you drink more fluids then you are cleansing your kidneys and encouraging them to become more efficient at clearing the fluid from your body. You also excrete excess water from your bowels when you pass stools, through the skin when you sweat and via the lungs during respiration as carbon dioxide.