If you ever see an aqua fitness class you will notice that no one taking the class is lean and has muscular definition. This is because the body is more buoyant in water. The body floats in water and this regulates muscle metabolism (cooling effect). Fat is not released at high rates because the body is not working as hard as it would if it were on land. In order to get burn fat it is essential that the body be placed under resistive forces that feature both eccentric and concentric contractions. The concentric contraction is responsible for the shortening of the muscle, the eccentric contraction is the lengthening effect of the muscle. It is during the eccentric contraction that most strength gains are developed. The eccentric contraction producing great fatigue to the muscle if performed slowly and deliberately. In order to force new growth patterns to create new muscle tissue the muscle tissue must be broken down. Most of the damage to muscle is produced during the eccentric contraction. The concentric contraction produces a breakdown of muscle but it produces a greater endurance to force. This is why swimmers can sustain longer durations swimming laps. If they were working eccentric contractions in addition to concentric contractions they would not be able to last as long. They have stored energy in the muscle that would normally be used for the eccentric contraction
I have never seen a swimmer have the muscularity of a sprinter or a bodybuilder. The only ones that seem to have muscular development from swimming are the hard core olympic level competitors. Even these athletes don't look like a land athlete like a world class sprinter. The swimmers body is more smooth in definition and the muscles appear long and stringy. The fatter a person is the more buoyant they are and this makes it very hard to metabolize off any fat. It is like wearing floaties and expecting to lose fat. Swimming does provide a nice therapeutic effect for the joints and also provides a nice stress release, and it is a good cardiovascular exercise. But to use a pool to lose body fat is a waste of time. You are must better off performing exercises on land that require resistance. Weight training is the greatest fat burner of all. So before you enroll in an aqua fitness class you might want to consider entering a program that is going to actually change your body composition. Pools just don't match up.