According to SheKnows, a person at about 150 pounds will burn over 400 calories every hour swimming at a moderate pace. It is about the same amount of calories a person would burn during an hour long brisk walk.
Swimming is a great calorie burning, low impact exercise. It works both the upper and lower body and is excellent for those with weak joints. SheKnows states that the key to using swimming as a calorie burning exercise is slowly increasing the duration of swims. This allows the body to burn more calories each time. Unfortunately, swimming does not build much muscle because it is not a weight bearing workout.