After USS Pueblo and USS Liberty incidents, US SIGINT doctrine was change; all ship-based SIGINT missions would only be conducted by U.S. Navy destroyers equipped with a mobile "van" of signals intelligence equipment. These types of patrols were conducted off the coasts of the Soviet Union, China, and North Korea, but are widely recognized for their role in the Vietnam War. The USS Sphinx (ARL-24) was the exception to this rule; the ship went through a $25 million overhaul to become an intelligence-collection platform. It conducted patrols off the Pacific coast of El Salvador monitoring the actions of the communist guerrilla forces. The ship was decommissioned for the last time on 16 June 1989 at Norfolk and laid up in the Atlantic Reserve Fleet.
Base on this information, the US would not like build new Spy Ships from the ground up, but instead modified an existing naval vessel; like the Stalwart-class Auxiliary General Ocean Surveillance Ship (TAGOS);
http://en.wikipedia.org/wiki/Ocean_Surveillance_Ship; or lease a civilian vessel, though a dummy corpration.