Smugglers who charge them about $4,000 each for the illegal crossing often use two boats with different crews for the short trip, forcing them to change at sea, authorities say. That way, the hired hands will have less to tell if they are captured.
U.S. officials and academics suspect heightened enforcement on land is pushing migrants to gamble their lives on the kind of dangerous voyages - on flimsy watercraft and with little regard for weather - more commonly associated with Cubans and Haitians braving the Florida Straits.
"Anytime you put pressure on a point along the border, the traffic moves somewhere else," said Juan Munoz Torres, a spokesman for Customs and Border Protection. "The only thing left is the ocean."
A spate of recent captures and discoveries of abandoned boats off California's coast climaxed shortly after sunrise Wednesday with a dramatic example of the increased risks that migrants are taking.
The crew of a pleasure cruise saw people waving from a 24-foot skipjack adrift about 12 miles off the San Diego coast and 20 miles north of the Mexican border. The cruise ship called a private towing firm, which alerted authorities to the 11 men and four women aboard a boat meant to carry far fewer.
The 14 Mexicans and one Salvadoran told rescuers they had been afloat without food or water. Some were dehydrated and sunburned, but no one was seriously hurt or killed. Authorities say the boat was leaking water and hardly anyone on board knew how to swim. Officials say they initially believed the boat was stranded for three days, but later determined it was less than two.
"They repeatedly expressed how they feared for their lives and thought nobody was going to rescue them," said Lauren Mack, a spokeswoman for U.S. Immigration and Customs Enforcement.
Some of the people on the boat told authorities they left Playas de Rosarito, a city of 130,000 just south of the border, and changed boats at Mexico's uninhabited Coronado Islands, according to ICE. The engine died 20 minutes into the second leg of the trip.
Three Mexican men on the boat were charged Thursday with immigrant smuggling, punishable by up to 10 years in federal prison if they are convicted. Octavio Ruiz Gomez, Jorge Acuna Betancourt and Jorge Ames Barr were scheduled to be arraigned Friday.
All three defendants denied to investigators that they were the guides, though Acuna admitted leading people across the border illegally in the past, according to the statement of probable cause. He said he was offered $500 to be a co-skipper on a boat smuggling attempt 10 months ago.
Three Mexican passengers were being held as material witnesses.
Smugglers have ferried immigrants for years, often favoring summer months and daylight hours, when it's easier to blend in with fishing boats, sailboats and other recreational watercraft on San Diego's crowded waters.
Since last summer, however, U.S. authorities in San Diego have found about 20 boats apparently used for immigrant smuggling - some abandoned, some with people on board. Several boats have washed ashore in Del Mar, a small suburb of million-dollar homes, 30 miles north of the border, with expansive beaches and easy highway access.
No deaths or serious injuries have been reported in those attempts.
Most of the skippers had traveled at night without lights on lightweight boats intended for one-time use, a departure from past practice of relying on more seaworthy vessels, said Mike Unzueta, the lead ICE investigator in San Diego.
The hired skippers are sometimes down on their luck. Scott Leo Paul, 51, had been unemployed and homeless for much of his adult life when he agreed in June to guide five Mexicans into San Diego Bay, home to several military installations and downtown skyscrapers, according to court documents. He pleaded guilty to immigrant smuggling.
Passengers told authorities they were transferred at sea to the 16-foot vessel that Paul helmed. They agreed to pay $4,000 to $4,500 each for the trip - about two to three times what it normally costs to be led on the much longer journey through mountains or deserts.
The smugglers rely on drivers who shuttle migrants to safe houses once they reach U.S. shores, authorities say. In November, authorities said, they arrested Dale Stamper, a U.S. citizen, as he steered five illegal immigrants from Mexico on a 19-foot boat toward Mission Bay, home of SeaWorld Adventure Park.
Authorities also arrested two illegal immigrants who were to meet Stamper at a public restroom and "help the aliens get into a transport vehicle," according to a federal complaint. One said he took the job to settle an $1,800 debt with smugglers for crossing the border a day earlier.
CBP added 39 agents to patrol U.S. waters last year, nearly all of them in San Diego and Brownsville, Texas, near the Gulf of Mexico. That brought its marine force to 150 - most of them assigned to patrol the Caribbean and Florida.
The Coast Guard also patrols the seas for illegal watercraft, but it's an uphill battle.
"It's a wide open Gulf and it's a wide open Pacific," said Nestor Rodriguez, a University of Houston sociologist.
Authorities suspect this week's mishap may give smugglers pause, though not enough to deter them.
"These arrests may cause them to step back for a while, but they're driven by money and greed," Unzueta said. "They'll be at it again."