When U.S. Immigration agents found a boatload of Mexican castaways huddled on a Malibu beach, and a larger group marooned a week later on an island north of Los Angeles, they realized it was a new trend.

Tightened security along the U.S.-Mexico border is pushing human smugglers from Baja California to forsake the treacherous trek across mountains and deserts in favor of ferrying illegal immigrants by sea.

"They're going farther north," said Jerry Conlin, a spokesman for U.S. Customs and Border Protection in San Diego, Calif. "These smugglers will take would-be crossers to the most remote areas where they believe their chances are much greater at successfully smuggling these individuals into the U.S."

U.S. authorities have tallied 12 incidents of human smuggling by boat off Southern California in the first six months of 2011, compared with four in the same period last year, according to Immigration and Customs Enforcement.

ICE attributes the increase to stepped-up patrols, fencing and camera surveillance along the nearly 2,000-mile (3,200-km) U.S. border with Mexico.

In response, agents have added boat patrols and surveillance units, and are collaborating with the Mexican government.

In one striking case this month, 15 Mexican and Guatemalan nationals had to be rescued from Santa Cruz Island, about 20 miles off the coast of Ventura, California.

Part of the Channel Islands National Park, Santa Cruz is a popular outdoor spot for Los Angeles-area campers, kayakers, snorkelers and hikers. But the illegal immigrants found there on July 8 had been marooned without food or water for three days, immigration officials said.

Authorities say the seaborne smuggling crews, complete with boat captains, first mates and navigators, typically transport illegal immigrants on rickety fishing boats.

They travel out to sea in the dead of night with no lights or safety equipment and drop their human cargo off on Southern California beaches. Agents so far have investigated such incidents in Orange, Los Angeles and Ventura counties.

Ten suspected illegal immigrants were detained on June 29 near the upscale seaside enclave of Malibu. A woman in that group had a broken nose, and one man had broken his leg, injuries suffered when they came ashore, officials said.

Agents caught 14 people at Crystal Cove State Park in Orange County on July 12 after their boat capsized. Three of the people arrested were charged with conspiring to bring illegal immigrants into the country.

One passenger, Sergio Carillo-Vasquez, told investigators that he agreed to pay the smugglers $7,000, according to a Border Patrol affidavit.

Claude Arnold, ICE special agent in Los Angeles, said it was surprising that no one is known to have died at sea.

"The smugglers don't care about these people," he said. "They look at them as a commodity."