I had my boat in Annapolis (Back Creek) for the previous 3 winters with a bubbler. I had a bit of water in my bilge from the stuffing box leaking slowly. The bilge water never froze. The brackish water in the Annapolis area coupled with the relatively warm water the bubbler keeps against your hull should prevent freezing in all but the worst winters.
Since the seacock is not closed the ice has somewhere to go when it expands and will lessen the likelihood of it cracking your seacock.
I assume no responsibility for any loss or damage incurred from listening to me.