The Soviet Union. Due to their relative lack of electronic listening posts overseas—in comparison to the Americans, who possessed signals intelligence (SIGINT) facilities throughout the world—the Soviets initially took the lead in the use of ships to gather intelligence. From the 1950s, they began using what came to be their preferred intelligence-gathering craft, a fishing trawler. The design of the trawler, which was made to store many days' catch in insulated compartments, made it ideal for extensive activities below deck.
As the Cold War continued, the Soviets expanded and improved their intelligence-collection ships, known to U.S. intelligence as AGIs, the AG being code for "miscellaneous auxiliary" and the I a designator of enemy craft. Later models were designed and built specifically to serve as collection platforms. Eventually they became large enough to include on-board intelligence processing facilities, greatly improving the speed with which raw data became usable intelligence for Soviet operatives.
During the Vietnam War, a pair of Soviet AGIs, one near Guam and the other in Vietnam's Gulf of Tonkin, kept a close watch on U.S. forces, and in some cases may have provided Hanoi with advance notice of U.S. airstrikes. Near the end of the Cold War, the Soviets had a fleet of about five dozen AGIs dispatched throughout the globe. A particular area of interest lay just to the east of Florida, in international waters and close to friendly ports in Cuba, from which Soviet AGIs could monitor activities at U.S. naval bases in South Carolina, Georgia, and Florida.