Migrant smugglers are skirting heightened security along the border by using small boats to shuttle people from the under-supervised Baja coast into Southern California marinas and harbors, already jammed with legal commercial ships and pleasure boat traffic.

In San Diego, across the border from Tijuana (search), federal authorities so far this year have seized 12 boats carrying 48 Mexican and Chinese immigrants, compared with three vessels with 20 immigrants for all of last year.

Further north in Los Angeles, the seizure of a French-built yacht crammed with 50 undocumented Mexicans and piloted by two Americans shed the most public light on what has been a busy summer on the water.

The trend has raised questions about border security. Officials have increased restrictions on land traffic and commercial sea cargo. But small pleasure boats can often slip in under the radar.

"If you can get across that border in a boat undetected, you can bring terrorists, people, weapons of mass destruction, who knows," said Special Agent Derek Benner, group supervisor of the U.S. Immigration and Customs Enforcement's San Diego Marine Task Force (search).

Mexican ports have been working with U.S. Coast Guard officials to meet new international anti-terrorism and security requirements that took effect July 1. The new protocols require each major harbor to hire a security chief who reports to the Transportation and Communications Secretary in Mexico City.

But smuggling gangs simply sidestep major Mexican ports, picking up migrants along empty stretches of coastline. And Mexican authorities say they can't stop potential smugglers because they don't actually break the law until they cross into U.S. territory.

Jose Luis Rios, director of the port at Ensenada (search), 75 miles south of the border, said maritime officials sometimes stop crafts with U.S. registration — but he said there's little they can do unless some other law has been broken.

"You can ask where they are going," he said. "But you're subject to their good faith — you have to take their word."

Smuggling syndicates work to avoid detection even inside Mexico because they often are moving migrants from other countries. Larger boats docking at harbors are also required to provide records on where they have been and where they are going.

"Are Mexican ports safer? Totally and absolutely," said Reymundo Mata, Mexico's adjunct director of marinas. "But gangs of organized criminals on the high seas, they probably won't be detected."

Authorities have not captured a single migrant-smuggling boat near the Baja California coast in recent memory, according to Elizabeth Juarez of the federal Attorney General's office.

U.S. authorities say Ensenada was the starting point for the largest U.S. maritime smuggling case in years — the Aug. 30 apprehension of the 44-foot C'est La Vie sailboat near Los Angeles Harbor.

U.S. Coast Guard officials acting on a tip intercepted the yacht after it failed to stop for inspection. Its cabin was designed to sleep eight, but crowded inside were 50 Mexicans, including three pregnant women and three children.

Two Americans jailed in the case told authorities they rented the C'est La Vie in Los Angeles and sailed 24 hours to Ensenada, where they picked up their human cargo.

Kevin Jeffery, deputy special agent in charge of Immigration and Customs Enforcement in Los Angeles, said the suspects rented vessels at least three other times in recent weeks and that Long Beach Harbor (search) may be becoming a major drop-off point for illegal migrants.

"We are not certain it's these two individuals, but we have indications that there is smuggling activity going on," Jeffery said. He added that houseboat residents in the area have "heard the pitter-pattering of many feet" on the docks late at night.

The C'est La Vie was registered in the United States and was required to check-in with Mexican officials upon reaching Ensenada. But Rios said his port has no record of the yacht.

"Officially, it was never here," he said. "We are investigating to see if it was allowed to enter improperly. For now, that's speculation."

Benner said cases in San Diego often involve undocumented Mexican boat captains who steer small vessels with U.S. registration into crowded beaches and ports, attempting to blend in with fishermen and boaters.

"The boats they use are $2,000 to $5,000 throwaway boats," Benner said.