Swimming is one of my favourite activities so I practice it three time per week but, if you are not accustomed to swimming, the first few days your body will adapt and one of the adaptations is to create more muscle mass to make this exercise more effective. This will make don't note the fats burned because you will be increasing your weigh with the muscle mass.
Normally when you do an aerobic activity such as swimming the percentage of fat burned is greater when you take a few minutes in the activity, this means you should swim at least 15 minutes with a medium intensity.
A mistake in wanting to lose weight by swimming is swimming at a high intensity, it makes the majority used energy are carbohydrates instead of fats.
Don't worrt if firsts days you can't swim more than 5 minutes. Be patient, wait some minute to recover some energy, and continue some more minutes swimming.
Over the days you will be getting more fit and be able to take any more time swimming, so it is also important to take it slow and go in progression.