Many people don't realise that most of your calories that you burn is only after your training. You go for a hour swim if you can make that, then have your shower and eat everything you see. This is totally off the track. If you want to effectively burn calories swimming, you need to not eat an hour after training.
Although you aren't still working, your metabolism is. It's working behind the scenes where you don't even realise it. Eating away calories and fat that if you ate during this time when add and not benefit in your losing calories.
So next time you have a swim, stay away from food for awhile and burn more calories.