I really don't know the answer to your questions but if you do the same exercise all the time, after about 4-6 weeks it stops it's benefits, you reach a plateau. You need to either start swimming for longer periods, more intensely, or just change your workout all together by doing some other type of cardio, like walking or running, biking. You should also do some weight training to supplement your workouts...
I also found
this article which explain very well why we don't burn as many calories swimming as other activities...
Quote:
One thing swimming is not good for is losing a lot of weight. The on-line Merck Manual explains that swimming is not the best way to lose weight due to the cooling effects of being in the water. While you do use a lot of calories swimming, once you get out of the pool much of that calorie burning stops. Doing land based exercise like running or cycling may use about the same amount of calorie per hour as swimming, but once you stop exercising the land-based workout usually leads to continued increase in calorie use for as long as 18 hours after the workout. Why? Because when you are in the pool you don't heat up as much as you do on land, and your body does not have to work to cool you down as much once the exercise session concludes. Swimming does exercise almost the entire body - heart, lungs, and muscles - with very little joint strain. It is great for general fitness, just not a great way to drop excess pounds.
Maybe you could start using swimming as a cross training exercise as mentionned in the article...
Hope this helps, good luck keep us posted on your progress...