I have my boat shrink wrapped every year and, except for a few dribbles across the deck from rigging and mast penetrations, the deck and cockpit pretty much stay bone dry. The shrink wrap is well vented, I keep the sink seacocks open and the port lite in the head open for air circulation in the cabin, leave all cabin lockers empty and open, and distribute six Damp-Rid buckets plus seven H2Out desiccant cartridges throughout. With all that, I still sometimes get an inch or so of water in the bilge, occasionally some in the engine compartment too. It’s condensation from rapid temperature and humidity changes. There is just no way to avoid it, but the Damp-Rid and desiccant cartridges definitely mitigate it’s impact. I’ve never had any mold, mildew or even stale odor during winter storage.
While I agree that ice is not likely to to do harm in a open bilge, I still like to leave a little antifreeze in the bilge to protect the pump. I might be wasting some antifreeze, but it’s cheap insurance, and it makes me feel good that I’ve done everything possible to prevent any freezing damage.