you don't burn as much swimming because your body does not heat up as much when it's in the water. You get the same aerobic benefit, but not the same calorie burn

I think the main reason has nothing to do (directly) with how much your body heats up, but moreso based on oxygen expenditure. In fact, depending on the temperature of the water, and the intensity of your swimming, you may not be able to cool off fast enough just from the water surrounding you (not being able to evaporate sweat off of you). The types of muscle actions (and probably the type of muscle performing the actions) are different in swimming than in running, which probably plays a large role in oxygen consumption/energy usage. I did see evidence from a study on swimmers in different water temperatures - apparently the lower temperature of the water relates to a larger appetite after the workout - be careful not to eat too much, swimmers! Anyway...

I'd rather play soccer than run. It's one of two ways I can be tricked into running.