[cbloom]

Well let's not get too silly. Lots of exercises burn calories. If you eat a ton afterwards, guess what, you won't lose weight. Calorie differential = weight difference.

Swimming and cycling are both basically low intensity steady state cardio/endurance exercises and thus don't really build the big muscles.

Most people who do them seriously tend to push until their bodies have very low blood sugar, which is not the ideal way to workout for body composition change, you never want to really reach that "starving" state.

If you're just trying to look better you can of course do intervals of either swimming of cycling. A lot of people who swim a ton of laps will just get into a relaxed groove and have good technique and can just glide without a lot of effort; since you're not fighting gravity your calorie burn can be very low if you're not pushing.