What would be the lowest water temperature survivable by either Koior Goldfish?

I'm actually leaning towards the planned pond being occupied by
Shubunkins/Comets - the reasoning behind this being, that for now, I'm
not convinced that I would be able to build a large enough pond to house
koi.

Living in an area where we can get heavy frosts I need to carefully
consider temperature drops in the winter (although not to the extent of
those in even colder climates) - this winter has actually been very mild
but I can't count on this. Obviously the pond needs to be dug to a depth
below the frost line to prevent the whole thing from freezing solid.
From recent observations of the reservoir for my water feature coupled
with the depth of my old pond and how far that iced up I'm pretty sure
that I can get the depth to avoid a total freeze up.....but I am now
curious how low the temperature can drop at the bottom of the pond
without it adversely affecting the fish...

TIA
Gill