In the mid-2000s, when shipping stocks first became popular on Wall Street, the shares were commonly bought as a play on China's economy. China is pivotal to ocean shipping, whether it's container ships, oil tankers, bulkers or gas carriers.

"There's a saying that everything that moves out of China in containers has to come into China as raw materials," noted Oeyvind Lindeman, chief commercial officer of Navigator Gas (NYSE: NVGS), on his company's latest conference call.

Ominously, signs of China's weakening economy are showing up across all shipping sectors at once.

The glass-half-empty view is that pullbacks in shipping demand are bellwethers of more severe economic problems to come. The glass-half-full view is that declines are temporary. A rebound of Chinese demand for iron ore, oil and gas will eventually boost commodity shipping rates.