No, USCG Documented vessels do not need to be registered in Mississippi.
According to the Mississippi Department of Wildlife, Fisheries and Parks:
Boating Registration
Mississippi law requires all sailboats and every undocumented vessel equipped with propulsion machinery, whether or not such machinery is the principal source of propulsion, using the territorial and navigable waters of the State of Mississippi to be registered and numbered for identification.
No person may operate or give permission for the operation of any boat powered by machinery unless it is registered and numbered in accordance with the law. The owner of any vessel required to be numbered has ten (10) days from the date for acquisition to register the boat.
Online Boat Registration
Reference: https://www.mdwfp.com/law-enforcement/boating-rules-regs/general-boating-rules-regulations/
