Admiral (ret) Ham Sang Whi was in charge of the naval blockade of Cheju at the time of the Cheju April 3rd Insurrection. He said that the island was completely isolated by the Korean Navy and the US 7th Fleet's modern warships and there is no factual base for the claim that Soviet or North Korean vessels reached the island.
Ghosts of Cheju - A Korean island’s bloody rebellion sheds new light on the origin of the war