SAN DIEGO, Calif., (Reuters) - U.S. authorities are building a steel and concrete barrier 300 feet (90 meters) out into the Pacific Ocean south of San Diego to curb dangerous attempts by illegal immigrants and smugglers to slip through the breakers to California.

The new maritime fence is being built at a cost of $4.3 million at the point where the U.S.-Mexico border plunges into the ocean between San Diego and the industrial powerhouse of Tijuana, in northwest Mexico.

The new “surf fence” is a steel-and-concrete barrier up to 18 feet tall that replaces a rusted and uneven line of posts.

“It was falling apart, it was out of alignment, it looked like a bad set of teeth,” said Customs and Border Protection spokesman Ralph DeSio.

“This is going to be much more aesthetically appealing to that area, but it also strengthens our abilities to prevent those dangerous smuggling attempts along that shoreline.”

Federal authorities have in recent years added fencing and Border Patrol agents along the southwest U.S. border with Mexico in a bid to stop illegal immigrant crossings and drug smuggling to the United States. Congress also mandated building a further 650 miles (1040 km) of fencing along the 2,000-mile (3,200 km) border.

Recent attempts to slip north through the surf and inshore waters to Imperial Beach, south of San Diego, have included two smugglers nabbed with marijuana piled onto a surfboard in 2009, and a pair of wetsuit-clad illegal immigrants arrested last February with self-propelled underwater dive scooters. Illegal immigrants have also taken to the sea to swim around the existing barrier to Border Field State Park in southern California. One man drowned attempting the trip last November.

The upgrade comes at a time when smugglers are increasingly pushing further out to sea in open-topped “panga” fishing boats to run illegal immigrants and tons of marijuana up the coast as far as Los Angeles and Santa Barbara.