Hell yes swimming will lean you out! Check out the bodies of professional swimmers like Dara Torres, and even better: read up on how much they have to eat to maintain their weight.
Swimming works large muscle groups at a time and is zero-impact. The best way to use it as a cardio work out and burn away body fat is to swim intervals/sprints, or swim a mile to make a benchmark, then try to beat it every time. You will notice if you swim daily and really push yourself, your metabolism will increase and you will lean out a lot - especially in the abdomen/obliques area and chest/shoulders. The last time I trained for a competition I lost 14 lbs in two weeks, and I almost never stopped eating! You have to swim hard though, like with anything else, if you're relaxed, you're not doing it right