I don’t know enough about what a Navy anti-submarine exercise is to determine if this had any relevance:

The night a torpedo-armed North Korean submarine allegedly sank a South Korean patrol ship, the U.S. and South Korea were engaged in joint anti-submarine warfare exercises just 75 miles away, military officials told The Associated Press.

The blast that sank the Cheonan, the worst South Korean military disaster since the 1950-53 Korean War, showed how impoverished nations such as North Korea can still inflict heavy casualties on far better equipped and trained forces, even those backed by the might of the U.S. military.

Defense Secretary Robert Gates said Friday that plans for more joint U.S.-South Korea anti-submarine exercises, announced after the sinking of the Cheonan, are on hold awaiting United Nations action on the incident.

In part, Gates said, there is concern about instigating another rash act by the North Koreans.

Two months after the sinking, U.S. officials for the first time disclosed details of the joint naval exercise held the same day as the attack on the Cheonan. Forty-six South Korean sailors died on the warship, which was not involved in the exercise but on routine patrol near disputed waters.  [Military.com]

My first thought when reading this was could the sonar even pick up a small, slow moving diesel sub in shallow waters from 75 miles away?  Fortunately the article answered my question:

Underwater, tracking submarines relies on active or passive sonar. Passive sonar uses microphones to listen for the sounds of sub operations. Active sonar emits sounds and listens for the echoes as they bounce off of submerged objects.

The Cheonan was operating its active sonar at the time, South Korea’s Navy officer Kim Young-kyu, a spokesman for the U.N. Command in Korea, told The Associated Press. It wasn’t clear why the ship didn’t detect the sub.

After the blast, a South Korean commander dispatched a patrol boat to look for subs.

But officials said the vessel couldn’t locate any, perhaps because of the weather, currents and rough conditions that chilly March night. Those factors, as well as the rocks and ledges in shallow water, can all affect the reliability of sonar, experts say.

Sonar technology has traditionally been designed to operate in deep waters and used for convoy protection rather than coastal defense.

“There’s a lot of equipment that works pretty well against big submarines out in the deep ocean, but doesn’t work so well against small submarines in shallow water,” analyst Pike said. “We’ve got the same concern with Iran and the Persian Gulf.”

So in other words the anti-submarine exercise going on was irrelevant to what happened because their sonar couldn’t have picked up the submarine that far away especially considering the Cheonan’s sonar didn’t even pick up the submarine.