S.C. Code Ann. § 12-37-3200

The tax year for boats, boat motors, and watercraft subject to property tax pursuant to Section 12-37-714 begins with the last day of the month in which a certificate of number required by Section 50-23-370 is issued and ends on the last day of the month in which the certificate of number expires or is due to expire. No certificate of number may be issued for a boat, boat motor, or watercraft until the ad valorem tax is paid for the year for which the registration is to be issued. All ad valorem taxes on a boat, boat motor, or watercraft are due and payable one hundred twenty days from the date of purchase.