I have had luck with some tropicals that normally wouldn't survive by
putting them in the deepest part of my pond before winter. I am in
Zone 5 (Chicago) and use a heater to keep the water from freezing.
You'd be surprised how many tropicals/sub-tropicals can survive in the
30s. Freezing is what will kill most. With my cover and heater and the
ground as an insulator, the bottom of the pond is probably always at
least 38 degrees.