A video of the incident, posted online Tuesday by one of the beach-goers, showed the amphibious landing craft "Zubr" noisily creating huge waves as it approached a crowded beach in Mechnikovo, TV Novosti reported Wednesday.
Russia's Defense Ministry said the ship was on a regular drill and that the beach was military property.
"Docking at the beach is a regular practice; what we don't know is what people were doing at the beach, which is within the military firing range," a ministry official told Komsomolskaya Pravda.
A military representative said the ship was barricaded to ensure the security of the beach-goers.
"After the drill was over, the cordon was removed and the ship sailed back to its base," said Andrey Bespaly, a public relations representative for the Baltic Fleet Western military district.