David Boykin steered his bateau right over a chain-link fence and looked glumly at his new home. It was surrounded by a lake that had flooded every house in sight. Boykin's house was sitting in about two feet of water.

"I just got finished building it," he said. "I was just fixing to lay the carpet and move in this weekend."

A programmer analyst at a Baton Rouge bank, Boykin and his wife, Cheryl, abandoned the house around 1 a.m. Friday after struggling in vain to keep the waters from the distant Amite River away from their door. Boykin moved his aluminum outboard back out onto state Rte. 447 and glided past Zeb Jones' high-standing house.

"Zeb put it on concrete blocks seven feet high and there's still three inches of water in it," said Keith May, a neighbor who joined the tour in his own boat with his cousin and business partner, David May.

Two colonies of fire ants floated by. One was the size of a football. The two reporters aboard, along with Joseph V. Colson, director of the state's Office of Emergency Preparedness, kept a lookout for snakes.

"Mainly water moccasins," David May explained. "Chief killed one about an hour ago--with a boat paddle."

The devastation here seemed far worse than in Denham Springs about a dozen miles to the northwest, where Louisiana Governor David C. Treen had been so shocked during an inspection tour on Friday.

The river itself was nowhere in sight. Its banks were about two miles away. But the flooding from torrential rains this past week spread far over the low-lying flood plains of Livingston Parish, where about 40,000 people live.

The rains claimed at least 10 lives and forced about 27,000 people to flee flood waters in Louisiana and Mississippi before tapering off today. The storm moved into Florida, however, and spawned tornadoes that killed three persons and damaged more than a dozen homes and businesses in Citrus County.

White House spokesman Larry Speakes said President Reagan telephoned Gov. Treen and Mississippi Gov. William F. Winter to assure them of federal aid to augment state and local relief efforts.

The Amite River crested at nearby Port Vincent around 6:30 this morning, but by then most area residents had been forced from their homes. Dozens stood hopelessly this afternoon at the edge of Rte. 447 before it disappeared underwater a few miles south of Interstate 10.

One resident, Elaine Walker, looked at her $10,000 mobile home in the distance as she handed out Polaroid snapshots documenting the damage.

"It's 3 1/2 feet from the ground up to the floor of my trailer," she said. "And the water's 19 inches deep in the house . . . . I will not stay here no more."

The Boykins' distress was especially poignant. They had been living in the mobile home near the new house he built, but that too was drenched. A Holstein bull he had been keeping vanished and was presumed drowned. His new truck was stuck in about four feet of water.

The boats turned right off Rte. 447 onto Vincent Place Avenue, now a wide stream. "The Amite River is over that way, about a mile to the south through those woods as the crow flies," Keith May explained. Both he and David May said their homes were flooded along with a heating and air conditioning business they run. Keith May said he had had flood insurance but gave it up after the severe floods in 1977.

"All the insurance did was pay the bank off and the bank took the trailer," Keith May said. "I figured it was a freak thing, that flooding like that would never happen again."

The flood waters here were receding slowly this afternoon. But although the rain had stopped, waters were still rising in the southeast corner of the state near Slidell, where the Pearl River was beginning to pose a serious threat. National Weather Service meteorologist Jim Belville said that the previous high-water mark for the Pearl River gauge above Slidell was recorded in 1874.

"We exceeded that level early today and the water is still rising," he said tonight. "We are dealing with something people here have never seen before."