Will putting a big container of water
in cold frame give off heat at night
It will of course work better if it contains warm water to start with.
If temperatures are likely to fall below freezing, it will effectively give off heat until the water in the container is completely frozen. How effective this is depends on how well your cold frame is insulated - is it double-glazed? wooden or metal frame?
Finally remember that after a cold night, the water will now be at low temperature and therefore reduce the day-time maximum.
I suppose success with this will be dictated by your location. This is the predominant source of heat for my house and many others here in Northern NM (except the heat storage is usually the cement slab, brick floor, etc...) because of the sunny climate. In the PNW, by the Great Lakes, in New England, and other regions, this is much less effective of course.
I have a metal rain catchment cistern in a SE exposure up against the house that rarely freezes and when it does never accumulates more than a few centimeters of ice, even in record -23 F cold. On the shady north side of the house I have had rain barrels freeze completely solid by the end of winter.