The Black Sea was originally a fresh water sea or lake. Sometime back, but in historic time, meaning man was in Europe, the barrier between the Mediterranean Sea broke through to it. Pouring in massive amounts of water that drained it and brought massive amounts from the Atlantic into it. Eventually all this went back some ways, and took awhile to settle down, like rocking a glass of water badly. Fresh water creatures can be found, at least their remains on the bottom of the Black Sea. It sets a date, as there are no salt water creatures that have been found that predecease them.

Some of the Central Americans have legends of the ocean going out, really far, and then sometime later washing back forcefully.

It might not explain China's or India's but will work for a good part of the world.