Swimming is a great aerobic exercise (cardio), and will help tone up your entire body since it uses upper body, lower body and core muscles. It is also a non-impact exercise and injury is rare. I have no idea what kind of cardio you are doing, but you could replace some of it with swimming, get some great cross training effects, and reduce your risk of injury. If you swim for exercise, swimming at an aerobic pace, breathing hard but not gasping for air, is what you want to shoot for. Like any new exercise, start slowly and build time and intensity as you improve. Swimming lessons are great for improving your stroke if it has been a while since you've been in the pool. And finally, respect the water, and swim with a lifeguard or at the very least a buddy.
Yes, swimming will make your arms and core look better, but the only way to get rid of fat anywhere in the body is to use it up, by using more calories than you take in, and it sounds like you are on the right track there. Also, if you are looking for muscular definition, you can probably get more of that (and more quickly) through strength training, which builds muscle in your arms.
Long enough have you dream'd contemptible dreams,
Now I wash the gum from your eyes,
You must habit yourself to the dazzle of the light and of every moment of your life.
-from Song of Myself, Walt Whitman