That's an interesting observation... I'm hoping fort the opposite! I was looking at a video of myself swimming and noticed there is a much larger frontal section presented to the water... aka, large gut hanging down. I'm hoping to reduce my drag my losing the weight!

Maybe you are no longer as buoyant.

Judging by the hairy overweight men and the pudgy women that fly past me in the pool, I'd say that fat floats and provides buoyancy and therefore makes a faster swimmer. Lose enough weight and you have to spend a considerable amount of energy just to stay on top of the water!

At least that's what I tell myself. Please don't burst my bubble...

Fat lost in your upper body actually might help you go faster. (Less frontal area, less plowing through surface tension at the front.) Fat lost in your legs definitely does not help you go faster. (Legs lower creates full-body plowing effect.)

Also, losing weight means you might still be at a calorie deficit. That could make your swims lose a bit of pop as well. Theoretically. It all depends on the athlete and day to day.

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I've heard that that's true. Lower body fat *might* imply leaner muscle mass (or a higher percentage of your weight coming from muscle). And since muscle tends to be more dense it takes more effort to stay higher in the water and reduce frontal drag. But yeah, as others have said, it's potentially a combination of things. As a silver lining, if you're seeing your power to weight ratio increasing on the bike that probably means you aren't sacrificing power with the weight loss and hopefully those gains would greatly outweigh a couple seconds per hundred lost on the swim.

Look at some pictures of Phelps at last year's Oly Games. He's prob around 5% fat, lean as hell but still very muscular. Losing weight certainly did not slow him down. Similarly, if you were to go any AG swim meet, you'd see kids who are thin as a rail but yet swim very fast. I agree with the poster who said you may still be on a caloric deficit which would cause your swims to be a bit slower.

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If you've had a dexascan and you know for a fact that the overwhelming majority of your weight loss was fat, then I'm ready to to attribute this all to bouyancy effects etc. that others are proposing above.

The simplest explanation is that it's really difficult to lose weight without losing weight from your muscles. You may have just lost some some muscle mass in your core or upper body and that has affected your swimming.

That may not be a pure strength issue, though. When you're kCal restricted and working hard to lose weight, your glycogen - the energy stores in your muscles - may not be fully replenished.
If you were to take a day where you laxed your nutrition plan and carbed up a little, and you swam faster the next day, I'd say that would support of this hypothesis.

Also, it's really easy to develop a form issue or bad habit that can cost you a second or two. If you've got a good coach watching you regularly for such signs, that would count against this hypothesis.