Swimming in a leisurely fashion is not good for weight loss - it is better than watching TV but that is about it. Swimming is not renowned for fat loss either, something to do with the water temperature.
If you want to swimm for weight loss you will need to do sessions of at least an hour at a time and you will need to swim hard enough to raise your heart rate to training levels - maybe do intervals etc. For swimming allow say 10 beats less per minute than for running. For myself at 43 I need to get my heart rate up to about 150 plus to do any good. Simply swimming 1km in 25-30 mins every day isn't really going to do a lot - but as I said earlier its better than watching TV and if its the only activity you enjoy........