In college I used to do 30 laps every nite and it definitely helped me drop some weight, however it was probably more helpful in stress relief. I found it to be very relaxing w/ the controlled breathing, being in the water, etc. Even at my best pace, I never found it to be as aerobically productive as a good HIIT session. It isn't weight bearing, but that can also make it a non-impact form of exercise which is nice if you do have issues.
Here's a little excerpt from bodybuilding.com about swimming as a form of cardio:
But, swimming does have a few disadvantages, though. First, it is not particularly convenient if you don't own a pool or live near water. It's not like walking where you can walk out of your house and just do it. For maximum effectiveness you have to know how to swim, which can take a long time to learn properly. It is also a non-weight bearing activity and will not contribute to bone density, as other weight-bearing exercises such as running will.
Also, swimmers tend to carry more of their bodyfat subcutaneously (under the skin). This is the body's reaction to training in water. Because you are constantly losing heat to the water, the body will try to protect against this by increasing the insulation near the skin (fat). If you are trying to get very lean, regular swimming is not the way to go, though it is okay for occasional use.