I've never been diving in Korea although I know that some does occur there. I'm not sure how close a US base would be to it though. Most of what I've heard about happens on Cheju island off the south coast. I looked at one website for a dive op there once. It was run by a German and the dives didn't look too bad - similar to Japan. It's probably quite hard to get to though for just a 'quick' dive.