Dangerous Dive

When the government of Guangdong province in southern China launched a campaign in 2002 to clean up the Pearl River, officials memorably promised that within three years the water would be "neither black nor stinking," and completely clean by the end of the decade. China's environmental regulators have been inching towards those goals: last month, after a government campaign called Operation Green Sword cracked down on more than 80 companies that were allegedly polluting the waterways around Guangzhou, provincial authorities triumphantly declared the Pearl River safe for swimming.

But a government-planned swimathon this week, which organizers hope...