You should really wait for all the ice to go and the lake to get up to about 50 degrees or above. That way the water acts as a heat source to those cold night below freezing. The air temperature around the waters edge will be about 10 degrees warmer than inland during short periods of freezing weather.
Worst case is that the pipes (and pump) might freeze up but won't get so cold to do damage. Just have to wait for them to thaw before you can flush.
