If this was at the famous place near Dandong by the Hushan Great Wall, where the distance between Chinese soil and North Korean territory (a farming island) is only about 5-6 meters, then I'm afraid it's not possible anymore. Foreigners and Chinese used to get a great kick out of wading across the little shallow tributary of the Yalu River into the North Korean fields located on this island, then hotfooting it back. You could also do little small boat trips down this tributary.

No more. I was there last August and the Chinese have erected a metal wire fence with signs all across their side of the river bank, and have posted a couple of sentries to prevent anybody from trying this stunt. You can look but not touch. The old boats for the trips are rusting and abandoned.