I must say that I would be worryed about that heating part, because that could led to a situation in wich the air temp could be lower then the water temp, that is your turts are in warmish water but they are breathing freezing air, wich could lead to a deadly respiratory infection. I think that you should do a few alterations to your very cool project. First your going to dig their pond as deep as you can (not to make the water going really deep, but the pond itself) that would make it much more naturaly resilient to temperature changes. Also I francly think that you should find some way to shelter your pond, and a great idea would be to build a green house over it, it has to be nothing fancy, it just has to avoid the formation of a deep ice cover. Ali snapper are tough but lets not overdue it. If everything goes well they would be hibernating with no probs. If ice forms just break it here and there. By the way I hope your pond is very large in order to house more then one snapper.