Actually, I'm pretty sure the grotto pool at the Swan and Dolphin is not heated. I am not sure about the lap pool. All of the official WDW resort pools are heated, including both pools at the Contemporary.
I think the Grotto Pool at the Swan/Dolphin may be heated but the huge waterfall that continually recycles the water cools it off. We were there in November and the water was definitely cooler in the Grotto as compared to the other pools but not so cold as to be uncomfortable. However, when I swan over towards the waterfall area there was a noticeable drop in the water temperature and if you stand and let the water fall on you there was a real chill.
The 'Spring Pool' which is right next to the big Grotto Pool is definitely warmer. I've read that the waterfall cools the heated water off considerably, but it is still heated. The lap pools are warmer too, so you do have other options if you are concerned about the water being too cold.
Here is a great photo. It will give you an idea of just how big the Grotto Pool and the surrounding area is!