It's possible if you go to one of those sea ports in China.

Ships depart from; Shanghai, Lianyungang, Rizhao, Qingdao, Shidao, Rongcheng, Weihai, Yantai, Tianjin, Qinhuangdao, Yingkou, Dalian and Dandong Ports to South Korean cities such as Inchon, Pyongtaek and Mokpo.

Go here for more information: http://www.travelchinaguide.com/cruise/china-korea/

If you want to go to North Korea, go to Dandong and swim or run across the bridge. You better be quick though' hehe

I met a guy back in Oz who had a job up on the river between China and Nth K. Said he had swum across the river a coupe of times and back with no issues. His version is: The Nth K's are more worried about their people leaving than others coming in