Based on the UNESCO description(s) China and Korea provide for their tidal flat nominations, the Wadden Sea is the exceptional inscribed site they aspire. Meanwhile, both sides seem to downsize the other in pursuit of their own world heritage aspirations. If this was Europe it would be a transnational nomination, but that is highly unlikely here.

Korea
- "In terms of the scale of tidal flats and ecological diversity, the Korean shore stands high above its Chinese counterpart."

China
- "Compared to the coast of the Bohai Gulf and the Yellow Sea of China, the Korean coast lacks great rivers such as the Yellow River and the consequent vast continuous mudflats."
- "The Bohai Gulf-Yellow Sea coast in China support more endangered species than the Korean side."

Great article about the tidal flats in China and Korea. I think one could make the case, ideally these nominations should be a transnational property between South Korea, China, and North Korea.

There is no mention of Yubu-do in the UNESCO descrption, but in recent articles it seems this important ecological site will be included in Korea's nomination.
"Yubu Island, the world's single most important site for the Spoon-billed Sandpiper."

http://www.birdlife.org/worldwide/news/%E2%80%9Cwaterbird%E2%80%99s-paradise%E2%80%9D -shortlisted-world-heritage-status