ritical water shortages and thick mud thwarted rescue workers yesterday as they struggled to unearth landslide victims and repair destruction from a storm that killed at least 23 people and caused an estimated $200 million in damage.

President Reagan declared five northern California counties-- Contra Costa, Marin, San Mateo, Santa Cruz and Sonoma--major disaster areas, making them eligible for help from federal agencies.

Officials in Ben Lomond, 10 miles north of Santa Cruz, said it could be days before they move the slide debris and reach the canyon homes that lie beneath it. The 1,500-foot-wide slide hit about 2 a.m. Monday and, witnesses say, it was so quiet that they did not awake. The mud covered at least a half-dozen homes.

"We have confirmed deaths in there, but we haven't been able to recover the bodies yet," said Santa Cruz sheriff's Sgt. Bruce Simpson. "We'll be working with crews with picks and hand shovels once we get in there."

Officials of the state office of emergency services said the storm and mudslides that followed destroyed 100 homes in Santa Cruz County, 70 miles south of San Francisco, damaged 300 others and displaced 1,800 residents. The main route into the county was closed to non-residents to discourage sightseers.

The storm also severed a 24-inch water main that supplied this city's reservoir, and officials said it could take a week to repair the damage. All industry and restaurants in Santa Cruz were closed to help conserve water.

"We can't take showers," Simpson said. "We can't eat hot food, travel is restricted, many people have been separated from their families, many of us working down here in the emergency operations center don't know how our houses are doing." Debris choked the city sewage treatment plant, sharply reducing the plant's capacity. Waste was being chlorinated, but was not completely treated before being dumped into Monterey Bay. Beaches were posted for possible pollution.

In Marin County, north of San Francisco across the Golden Gate Bridge, most phone service had been restored yesterday and commuting was near normal. A day earlier, the famous bridge was closed when a mudslide undermined U.S. 101, but engineers determined the highway was safe and it was reopened.

In southern California, meanwhile, biting cold Santa Ana winds slashed at 60 mph across the state, blacking out power in five counties and fanning a $1 million fire that destroyed five homes. The winds, forced through icy mountain passes from Nevada, knocked down power lines and trees in Ventura, Los Angeles, Riverside and San Bernardino counties.