The important thing with weight loss, particularly if you are obese, is to lose it slowly. You cannot guarantee you will not be left with loose sagging skin, because your skin has had to stretch alot to accommodate the weight increase over the years and may have lost suppleness and elasticity.
Walking and swimming is a good form of Cardiovascular exercise, which will help to burn calories and improve you Cardiac health and fitness. You do, however, need to do resistance training, which will help tone up muscles, help prevent sagging and increase your metabolic rate.
When you go out walking, wear a heartrate monitor, ensure you are working between 65 and 80% of your Maximum heartrate, this means your heart should be beating between 117 and 144 beats per minute. As you get fitter, you may need to work harder to maintain this rate.