After the fall of Sevastopol in June, 1942, the Soviet navy retreated to the east coast of the Black Sea.
North of the Black Sea is the much smaller Sea of Azov, with a narrow access through the Kerch Strait. The Germans controlled the two peninsulas on either side of the strait after the fall of Sevastopol.
Did the Germans deny the Russian navy access to the Sea of Azov, making it "safe" for waterborne transport by posting air and artillery units on either side of the Kerch Strait? If not (as I suspect), was that at least possible, given the technology of the time? (Later in the war, the Allies made the Atlantic, a much larger area, almost "submarine-proof" using air and search technology. Perhaps a better example is the fact that the Americans retained complete control over the Panama Canal.)