In the 1990s, thousands of Cubans cast themselves adrift on rafts, tires and any other makeshift craft that could float. They shoved off even from the island's far shore, entrusting their lives to the powerful Loop Current that pushes north from the Yucatán, rounds Cuba, rushes through the Florida Straits, and spills into the Atlantic, seeding the Gulf Stream. Many drowned or perished from thirst. But others survived to wash up on Florida's east coast. Now the Loop Current is in the news once again. Oil from the Deepwater Horizon gusher—please don't call it a "spill"—has begun trickling into the...