I used to use a floating heater in my pond over the winter to keep it from freezing over completely. Plants that are in pots should be fine as long as the roots or rhizomes don't freeze solid. After the heater fianlly died I didn't replace it, then last winter the entire pond was frozen solid for months. The water lilies all came up in the spring though, didn't have any fish left anyway. I had stocked it with rosy reds to eat mosquito larvae several years ago, but they finally died out the previous winter.