Shubunkins can live in very cold water. I have a friend with an outdoor pond with Shubunkins and there is ice on the top of it right now. If you prefer to have it heated, that is your decision, but they can survive in low temps. Keep in mind, there is a certain water temp where you aren't supposed to feed them, but I believe its below what you have. I think you need to look at other possibilities for their death. I'm sorry you lost some. My brother just lost all 6 of his because they ate rocks. Check the chemical levels in it, too.

Here is a site with more info that also says they don't need a heater... http://aqualandpetsplus.com/Pond,%20Shubunkin.htm

Here is a link that might be useful: Shubunkin Info

But it wouldn't harm them if I brought the water temp up to about 68 F ?

No, they won't be harmed if you brought the temp up to that. That is still cool... they can live in warmer temps than that.

I did bring the temp up to 68 and they love hanging around the heater. I may take it up to about 70 for them.