If you are in the frozen north, I would expect most plants to die back over winter, even the ones described as hardy.
I have got some bogbean in my pond, it seems to be one of the only things still around at this time of year. Though maybe best in a larger pond.
I also like frogbit. It is a kind of mini water lily, good for surface cover. Over winter the buds sink to the bottom of the pond. But it should return and spread as gets warmer.
Marsh marigold and water mint seem quite tough as well.