Check out this CBP article
for information straight from the horse’s mouth. Check the harmonized tariff schedule, duty is usually 1.5% but there are exceptions. I didn’t see one for ZA (South Africa) but was not thorough.
Read the information on EPA 3520-21, this can be a big gotcha, make sure any engines can be imported.
Sales/use tax will depend on the state of principal use, every state is different. Some have exceptions for time owned, some for taxes
paid in other jurisdictions, all depends on the state where you intend to keep the boat
PS Welcome to CF