The number of annual tests has dropped by nearly half since 2005, and some beaches are going untested for months at a time, the Los Angeles Times said, citing an analysis of state records.
The frequency that beaches have been closed by pollution dropped dramatically by 74 percent since 2005, and beach postings of possible contamination have dipped by 44 percent.
While massive water treatment efforts have made the water cleaner, there is concern that less testing means officials simply aren't detecting dirty water.
"Water quality absolutely has gotten better during the summer months," said Mark Gold, president of Heal the Bay, a nonprofit group that issues an annual report card on state beaches. "But the reality is that less frequent monitoring means there's a much greater chance of someone swimming or surfing in polluted water unknowingly."
Water contaminated with bacteria can subject swimmers, surfers and divers to skin rashes and ear, eye and stomach illnesses.
It is difficult to determine if less testing has caused more people to get sick. Public health officials said such illnesses are rarely reported.
In Long Beach, which historically has had some of the state's most polluted water, 40 percent of beach sites no longer are being tested, according to city officials.
In San Diego County, water at the world-renowned Trestles surf break in San Onofre State Beach was tested only four times last year compared to nearly 70 times in 2005.
A 1999 state law requires health officials to test beaches at least weekly during the summer, but only if the program is funded.
Some regions went to nearly year-round testing, but now some health and wastewater agencies say they have cut back testing because of state and county budget reductions.
Two years ago, Gov. Arnold Schwarzenegger vetoed $1 million that the state had provided annually for beach testing, but California has used emergency bond money and stimulus funds to keep up the testing while it seeks permanent funding.
The state continues to fund the mandated summer testing at a 90-percent level, according to the governor's office.
"It is not immediately clear why the number of tests taken by counties have declined this much," spokeswoman Rachel Arrezola said in a statement. "This is a concern, and the Department of Public Health along with the State Water Board are looking into the root of what is causing this."
Testing at nearly 40 Orange County beaches stopped for five months this winter because of a lack of funds.
"If there isn't a sign posted, I kind of assume it's safe," said Susan Thomas, who often takes her 16-month-old daughter to Baby Beach in Dana Point -- one of the sites with fewer tests.
"We're obviously taking a risk going into the water anywhere along this coast," Thomas said. "But when she swims, she goes under,"
Some agencies said testing remained adequate.
"We continue to do the tests weekly, and we're not doing less sampling because we don't have money," said Alfonso Medina, director of Los Angeles County's Environmental Protection Bureau.