San Diego ? The speedboat is about three miles offshore when a U.S. Customs and Border Protection agent cuts the engine to drift on the current in quiet darkness, hoping for the telltale signs of immigrant smuggling — a motor’s whirr or sulfur exhaust fumes.

“It’s like trying to find a needle in a haystack, and the haystack is the Pacific Ocean,” agent Tim Feige says, minutes before sunrise.

This is a new frontier for illegal immigrants entering the United States — a roughly 400-square-mile ocean expanse that stretches from a bullring on the shores of Tijuana, Mexico, to suburban Los Angeles.

In growing numbers, migrants are gambling their lives at sea as land crossings become even more arduous and likely to end in arrest. Sea interdictions and arrests have spiked year-over-year for three years, as enforcement efforts ramp up to meet the challenge.

While only a small fraction of border arrests are at sea, authorities say, heightened enforcement on land, and a bigger fence, is making the offshore route more attractive.

The number of Border Patrol agents doubled to more than 20,000 since 2003, and President Barack Obama is dispatching the National Guard after clamor for a crackdown in the desert led to Arizona’s tough new immigration law.

“I think they found that going west through the ocean is probably their best bet,” said Michael Carney, deputy special agent in charge of investigations for U.S. Immigration and Customs Enforcement in San Diego.

U.S. agents arrested 753 suspected illegal immigrants on Southern California shores and seas between October and Aug. 24, up from 400 the previous 12 months and 230 the year before. They spotted 85 watercraft since October, up from 49 during the previous 12 months and 33 the year before.

The smugglers use old, single-engine wooden vessels known in Mexico as “pangas.” They’re several feet wide and about 25 feet long. If they are found on U.S. waters, they’re almost invariably smuggling people or drugs.