The swim beach at Frank G. Bonelli Regional Park is a sand beach with a roped swim area on the south shore of Puddingstone Reservoir, which most locals call Puddingstone Lake. The park is run by the Los Angeles County Department of Parks and Recreation and covers about 1,800 acres of hills, picnic lawns and trails in San Dimas, just northeast of where the 57 and 10 freeways meet. The reservoir itself is a 250-acre lake behind Puddingstone Dam, which the county flood control district finished in 1928. The county has had lifeguards on the lake since the early 1930s.

The park address is 120 Via Verde Park Road. From the 57, take the Via Verde exit and follow the park road east. From the 10, exit at Fairplex Drive, climb to Via Verde and turn left into the park. Inside the gate, Eucalyptus Drive leads down to the swim beach and the paved lots beside it. The big Picnic Valley area with shade shelters, grills and playgrounds sits just south of the beach, so a lot of families make a full day of it. Raging Waters, the private water park on the north side of the lake, is a separate business with its own admission and has nothing to do with this beach.

The beach is a strip of sand along a sheltered bend of the south shore, and swimming is limited to the buoyed area in front of it. County lifeguards are on duty whenever the beach is open. The swim season runs from Memorial Day to Labor Day, Wednesday through Sunday, 10am to 6pm, and the county notes that actual operating days can vary, so call the park office at (909) 599-8411 before a special trip. Outside those hours and outside the ropes, swimming is not allowed, because the rest of the lake carries powerboats, jet skis, sailboats and anglers. The water is calm, with no current to speak of. A rental concession at the park offers kayaks, pedal boats and paddleboards.

There is no separate charge to swim, but the park charges a daily vehicle entrance fee of $15, or $8 for drivers 65 and older and for vehicles with a DMV-issued disabled placard or plates. Annual vehicle passes are $160 ($90 for seniors and disabled visitors) and are sold at the main office. Bonelli is closed every Monday and Tuesday. From March through October the park is open Wednesday to Sunday, 6am to 8pm, and from November through February it closes at 7pm.

The main hazard here is water quality. Lakes in Los Angeles County, Puddingstone included, can grow harmful algal blooms in hot summer weather. The county chlorinates its three swimmable lakes to cut that risk, but a state bloom warning means no swimming, so check the state's harmful algal bloom map or call ahead. The state also advises against eating fish from the reservoir because of mercury and PCBs. Dogs are not allowed in the water or on the sand beaches, and alcohol and smoking are prohibited. Because Brackett Field airport sits next to the lake, kites, drones and model aircraft are banned too.

Beyond the beach, the park has 14 miles of multi-use trails for hiking, mountain biking and horseback riding, an equestrian center, boat launches, fishing along the shoreline outside the beach and dock areas, and a privately run RV resort on the east side. The county also runs a junior lake lifeguard program here for ages 9 to 17.