Everything that reached Japan from the continent before the modern period came across this sea, and the religious cargo was substantial.
Buddhism arrived in the sixth century by way of the Korean kingdom of Baekje, whose ruler sent an image and scriptures to the Japanese court, and the debate at that court about whether to accept a foreign deity is recorded in the chronicles as a dispute between clans. What followed was not simple adoption but a long process of accommodation with the kami, worked out over centuries and eventually formalised in the honji suijaku framework, under which local deities were understood as local manifestations of buddhas. Chinese writing, continental ritual, temple architecture and, later, Zen and the Chinese schools all followed the same route.
Monks travelled in both directions and often at real risk. Ganjin, the Chinese monk who brought the full ordination lineage to Japan in the eighth century, attempted the crossing repeatedly over more than a decade, was shipwrecked and lost his sight before arriving.
The sea's name is itself a live dispute. Korea calls it the East Sea and has argued for decades in international forums that the current name reflects a period of Japanese dominance rather than long usage; Japan maintains that the name is long established and neutral. The atlas records the dispute rather than resolving it, and uses the more widely catalogued form for the row.