I live in Iowa too, my ducks fared the winter of 2019 fine, even when it got down to -35F. For mine, I use five-gallon heated water buckets, I insert them into the ground to help keep the water warmer, and so the water is level with the ducks, its a good idea to put a board over the top so they can't jump in, as there is always the chance they could drown since there is not much traction if a bucket. I've never used a heat lamp with full-grown ducks, because its not needed, they do well at acclimating to the temps, but if there were to be a power outage and the temperature in the coop dropped the birds may suffer from hypothermia, you also have to be careful with any heat source, because your coop will be at an increased risk for catching on fire.
My birds will roam as long as there isn't snow on the ground, The cold doesn't seem to bother them, but the snow does. Ducks don't need heat, but it is important you provide them with a draft-free area, thawed water/feed, and plenty of bedding, give them that and they will be fine.