Relaxing and sitting in a hot tub doesn't burn a whole lot of calories so you won't lose a whole lot of weight when sitting in a hot tube but you can burn off a few calories sitting in the hot tub.
You still need to watch your calorie intake and get exercise to lose a good amount of weight but the hot tub can relax your muscles and help your health in many other ways too.
When you sit in a hot tub for around 15 minutes you can burn off around 17 to 20 calories depending on your weight.
If you want to burn off more calories get out and exercise by jogging, running, walking using a treadmill, exercise bike, riding a bicycle etc.