ZAPATA, Texas -- The waters of Falcon Lake normally beckon boaters with waterskiing and world-record bass fishing. But this holiday weekend, fishermen on the waters that straddle the U.S.-Mexico border are on the lookout for something more sinister -- pirates.

Twice in recent weeks fishermen have been robbed at gunpoint by marauders that the local sheriff said are "spillover" from fighting between rival Mexican drug gangs.

Boaters are concerned about their safety, and the president of the local Chamber of Commerce is trying to assure people that everything is fine on the U.S. side of the lake.

At the fishing camp his family has owned for 50 years, Jack Cox now sleeps with a loaded shotgun at his feet and a handgun within reach.

In the American waters, Cox said, "you're safer, but you're not safe." Mexican commercial fishermen regularly cross the border to set their nets illegally, so why wouldn't gunmen do the same? he said.

This month the Texas Department of Public Safety warned boaters to avoid the international boundary that zig-zags through the lake, which is 25 miles long and 3 miles across at its widest point.

Authorities also urged anyone on the water to notify relatives of their boating plans to aid law enforcement in case of trouble.

Since issuing the warning, most boats have stayed on the U.S. side.

"That's a good indication. It means they're getting the message," Texas Parks and Wildlife Capt. Fernando Cervantes said Thursday as he patrolled with two other game wardens. "They're still coming out, but they're not going across."

The border is marked by 14 partially submerged concrete towers that mark the Rio Grande's path before the lake was created in 1954.

Game wardens and the U.S. Border Patrol watch over the lake but do not cross into Mexican waters, and no Mexican law enforcement is visible.

Men armed with assault rifles robbed fishermen on the Mexican side of Falcon Lake on April 30 and May 6.

They traveled in the low-slung, underpowered commercial Mexican fishing boats that are familiar here.

They asked for money, drugs and guns, and took what cash was available. No one was hurt.

A third incident happened a couple of days before the warning was issued, but Cervantes said the fishermen were able to escape without the thieves boarding their boat.

The attacks "were really unusual," Cervantes said. "We had never seen it, and then they started up."

Violence on the Mexican side of the lake has been climbing for several months.

A fractured partnership between the region's dominant Gulf Cartel and its former enforcers, the Zetas, plunged many of the area's Mexican border cities into violence.

Police stations were attacked, officers were killed, and rolling gun battles between the gangs and with the Mexican military became commonplace.

"To me, this is spillover violence," Zapata County Sheriff Sigifredo Gonzalez Jr. said. "I don't do the Chamber of Commerce talk. I talk reality."

Still, the sheriff said, boaters should be safe provided they stay on the American side.