At 0C or just below with no antifreeze you might get ice particles in the water which could collect somewhere (typically a joint or corner) and form a blockage which would reduce flow, it's unlikely it'd ice solid though unless you're talking major cold, I think 0F is probably major cold as it's -18C which is below what most antifreeze recommends. That is assuming the weater gets that cold given it'll be running over a cpu / pelt to keep it warm
