I don't heat my outside pond. I bring my turtle inside when the weather gets cold. I live in Texas and it gets cold enough here that RES living in the wild would hibernate if left outside. Hibernation is very stressful, even for a wild turtle living in a good wild habitat. Lots of them don't wake up in the spring. And it would be hard for one of us to create an artificial outside environment that would be as good for a hibernating RES as a natural one. The water has to be deep, with deep mud on the bottom and you'd have to keep it from freezing over completely.
The problem with trying to heat a pond - especially in a very cold climate like you must have in Michigan - is that the air is going to be way too cold. If the turtle tries to bask, for instance, he'd freeze. Another difficult thing would be keeping the pond water warm enough for an active RES. A temperature that's in the 50's or low 60's is called the "death zone" for a turtle because it's cool enough that his digestion slows down to nearly a stop, but not cold enough for him to hibernate. So, when the turtle eats, the food sits inside of
him and rots. You can guess the rest. I have a hard time imagining a heater sensitive and strong enough to keep the water temp above 70 in a Michigan winter, but not so hot that it would cook the turtle.
And I think it would be even more difficult for a Michigan person to create an environment for a turtle to hibernate in because you can't let it freeze solid - the turtle couldn't take it.
In short, I'd strongly suggest you look for ways to bring your turts inside for the winter where they would be safe. A nice rubbermaid container might be a good temporary home for them until it's ok for them to go outside again. A good pond heater might let you keep your turtle outside when it's kind of cool but not yet really cold.
By the way, RES do not naturally live in the really cold places like Michigan. they probably can't survive hibernation.
http://www.redearslider.com/basic_species_info.html