The ceremony came hours after North Korea said it had fired a newly developed anti-aircraft missile a day earlier.
Moon said, "The government and the military will sternly respond to any action threatening our people's lives and safety," without referring directly to North Korea.
Moon made the comments during a speech he delivered from the South Korean helicopter carrier Marado just off the coast of the eastern city of Pohang.
On Thursday South Korean armed forces staged amphibious landing drills at the bay where U.S. launched their first military landing operation during the Korean War, in July 1950.
North Korea has said in recent weeks that its weapons tests are aimed at boosting its defence capabilities just as other countries do, accusing the United States and South Korea of "double standards" and "hostile policy".