Well if you think about potential routes to China Koshima island with the world fameous Japanese Macaque population is close to the most likely to try swimming path between the Japanese island chain and China. Only real hang up is that it would be really easy to get caught by the shipping traffic in and out of Shanghai headed that way.
Take a look then zoom out a bit for the wider area. This little island is off the East side of the southern most major island in the Japanese Archipelago chain.