Is boat insurance mandatory is South Carolina?

Yes it is, liability will be required, or a bond must be issued to you in lieu of an insurance policy.

You'll have to check with your department of motor vehicles where you register the boat. You do have to have a title and registration for boat and trailer because of them being on the roads. If you find that you need an insurance policy you can get a quote from United Marine Underwriters. They are one of the largest companies providing boat, and personal watercraft insurance.