Water temperature is not a major concern where koi and goldfish are concerned, they are all very hardy fish and can sustain temperature swings when the spring comes around.
The temperature of water does not change as dramatically as air temperature, it tends to change a lot slower even if there is a quick cold snap in early spring, you will probably find the water temperature will hardly change at all.
If you do live in a severe weather swing area and still a little concerned, perhaps a pond thermometer or even a pond heater, this would stop any erratic swing, they do not increase water temperature in cold weather but would slow down any erratic drop, and this would give you peace of mind.
What do change are their eating habits, the colder the pond temperature the less they eat and grow due to their metabolism slowing down and being less active.
By feeding them when they are not eating very much causes other problems, the food stays in the pond and it can rot or decompose and then cause water contamination.
It is always better to feed fish with low protein in colder weather, even though they still eat they will not digest it all and they will then produce more waste. If you are not careful the waste can build up and produce toxins that are potentially dangerous to the fish.
Fish are quite hardy and capable of coping with temperature changes but not with contamination.
The two points to remember are, in warm weather feed high protein food to enhance growth, when cold, feed low protein as everything slows down.
Return from water temperature to water quality