I would be more worried about
the water outside of the hull freezing. If you think about it, water freezes from the top first. That is because the greatest expansion in volume takes place just above freezing. Note that the greatest ice pressures will occur when the water thaws not when it freezes. The water level in your ballast will be lower than the water level outside of the boat because of the boats weight and therefore the ballast water would be slightly warmer to start with than the ice outside (assuming the boat is floating in the water). If your boat has heat inside, it likely would not freeze and if it did, it would probably thaw sooner than the ice outside. That may be good or bad. The pressure from the ice outside of the hull will be unopposed unless the water inside were to freeze as well and even if it did, the difference in levels would result in some serious localized forces on the hull. If the freeze you expect is not too severe, you might use salt instead of glycol antifreeze in the ballast water. Just remember that salt is bad for fresh water life. You and Russ might read my response to engine antifreeze in another post re: toxicity.The issue of ballast freezing is much more complex than you might think at first.