Which act amended the Espionage Act of 1917 for better control of vessels?

The Magnuson Act of 1950 is the correct choice as it specifically addressed the need for better control over vessels in the context of national security. This act introduced measures related to the regulation of maritime activities, including those that could potentially compromise national intelligence and security. By amending the Espionage Act of 1917, the Magnuson Act enabled more stringent oversight of vessels, which was crucial in the geopolitical climate of the time.

Other acts mentioned, such as the Water Pollution Control Act of 1972 and the Clean Water Act of 1977, focus primarily on environmental and pollution control regulations rather than vessel security. The Harbor Development Act of 1965, while related to improving harbor facilities, does not directly amend the Espionage Act or focus on vessel control in security contexts. Therefore, the Magnuson Act stands out for its specific connection to enhancing vessel control with respect to national security.