Dennis Wilson, the drummer for the Beach Boys who rode the rock group's wave of popularity for two decades, drowned today, a sheriff's spokesman said. He was 39.

Wilson did not surface after diving into the water in this Los Angeles suburb, said sheriff's Sgt. Leroy Chastain.

"He and some friends were on a boat," Chastain said. "We do not know if they the friends were the Beach Boys or not. About 4:25 p.m. PST, 7:25 EST , he dove off the slip in about 12 feet of water and he failed to come out.

"They called in the Harbor Patrol and lifeguards and the body was recovered at 5:15 p.m.," Chastian said. "We do not know if they pulled out the body or if it floated to the surface."

The group was made up of Wilson, his brothers Brian and Carl, cousin Mike Love, and Al Jardine.

Earlier this year, then-Interior Secretary James G. Watt touched off a storm of protest from their fans--including Nancy Reagan and Vice President Bush--when he banned the Beach Boys and other rock groups from the District's Fourth of July celebration. In recent years, the Beach Boys' free concerts on the Mall and subsequent fireworks displays had attracted hundreds of thousands of people.

After his announcement, Watt was called to the White House for a meeting with President Reagan, and he revised his position. The Beach Boys played in Washington on June 12 for more than 50,000 people at RFK Stadium after a professional soccer game. On July 4 the group performed a free concert in Atlantic City for more than 200,000 people.

Dennis Wilson missed some shows during the Beach Boys' tour this past summer.

Wilson was the only surfer in the group in 1961 when the Beach Boys recorded "Surfin'," the first of their dozens of hits.

They got together while students at suburban Los Angeles high schools, playing mostly folk music and calling themselves "The Pendletones."

Murry Wilson, the brothers' father, was their manager. The youngest, Carl Wilson, was 15.

The surfing theme started when Dennis Wilson took up the sport. Love and Brian Wilson wrote "Surfin' " and watched it rise to No. 10.

With songs like "Good Vibrations," "California Girls," "Litttle Deuce Coup," "I Get Around" and "Fun Fun Fun," the Beach Boys cultivated a young, playful image.

As their contemporaries were moving into protest songs of the late 1960s and early 1970s, the Beach Boys maintained that their songs "didn't mean anything" and "their music was an experience," their public relations firm said in 1976.

Although the group was not particularly identified with protests or acid rock music, Brian Wilson entered extensive psychiatric therapy in the 1970s and admitted using LSD.

Dennis Wilson, once a friend of Charles Manson, was twice married to actress-model Karen Lamm.