Many people believe that in order to burn calories swimming they need to start out very fast and kick their legs off. WRONG! This will just wear you out before you can have an effective training session. The key is to start out slow and build your way up. When you in the pool and you see those kids at their swimming class doing so many lengths in an hour, they are burning plenty calories swimming. You however, are not likely there. They train for hours and hours to get were they are.The good news is you can get there. It will just take time.
Start out with an easy length then a medium, moving to a fast pace then a sprint. Keep your heart-rate at a bearable pace that you will not want to quit after the first length, but also not so slow that it takes 15 minutes to reach the other side.
To burn calories swimming, focus on effeciency rather than trying to improve those watching you. You will see results in the long run.
Build not lose
Remember that muscle weighs more than fat. You may not notice significant weightloss for awhile, but you will find that your muscles will strengthen and begin to look good.
Time is the key element, that and commitment. If you quit before you see the results you would have been wasting your time and you won't see yourself burn calories swimming.