SC does charge an excise tax, but as my boat was my home it cost me a tiny fraction of what it would have been if it wasn't. I seem to remember (but not for sure) that one could also get the discount if the boat was considered a second home (summer residence).
As for California, they are the greediest when it comes to boat tax. In Hawaii I traded my Phil
Rhodes ocean racer for a Wm. Hand gaff ketch built in 1909, a grand old lady for sure.
California began sending me a tax bill shortly thereafter. I sent them a nice letter explaining that the boat had not been to California since I'd owned it and that I was a Hawaiian resident. Another tax bill arrived, even bigger (penalties I guess). After about a year of these monthly bills, getting bigger each month, my responses were much less pleasant until the final one which I answered with the "F" word every other word. That seemed to be the magic letter, as they stopped sending the bills.
I couldn't believe they had the greed to try and intimidate me into giving them money for a vessel that hadn't been in their state nor was owned by a resident, for years. If you are a California resident I feel sorry for you, but don't just pay a tax bill without checking that you might be entitled to some exemption that they won't tell you about.