You might be wondering when you get in the pool what would be best to burn calories by swimming? Start swimming at your normal speed then gradually start slowing down, because you tired, but keep going thinking this is the best way to burn calories. Or find a good training program that works in intervals, some sprints, mediums and some long distance swimming.
You actually will burn more swimming by doing the interval training. Making you heart rate change, go up and down then up again. This is the best training that you can do.
Now, I'm not saying there isn't a time for long distance swimming or swimming until you can't anymore, but this should not be your main goal if you out there to lose those calories swimming.
It's very similar to going for a run, if you run once a month very hard you will definetly feel that you worked, but would it not be more effective if you run a little bit every day. It's better to burn 100 calories a day for a month than a 1000 in a month.
I hope this helps you understand the best way to burn calories swimming. There are many traing programs out there, find the right one for you.