The person I spoke to in the tax assessors office said all marina's and storage
facilities are required to provide a list of boats in their yard on 1 Jan. A yard manager told me his interpretation was "any boat regularly stored and used" in the county, but the folks in the tax office didn't mention that specifically.
Bills are sent out in the summer based on the NC taxable property as of 1 Jan - which she called the assessment date. She said there was a penalty if you did not declare your personal property. I was also told each county has the authority to determine how personal property tax is managed, but most counties follow a similar process.
I live in Virginia in Northumberland County, and all that seemed to be similar to my experience here (if you buy a boat on 2 Jan 2020, you don't owe the county's personal property tax that year). Since those rules sound similar I assumed I was getting the correct info.