Follow 580 to Castro Valley, about 20 minutes from Oakland, and you'll find the Cull Canyon Recreation Area, including a sand-bottomed swimming lagoon with lifeguards on duty.
My daughter tells me the main thrill is swimming to the far end of the reservoir where a large floating dock allows you to climb up and then jump off into 11-foot deep water.
What to expect at Cull Canyon Regional Recreation Area
Cull Canyon is an East Bay Regional Park. Parking is free at this spot, but the gates are only open for specific hours, 8 am to 10 pm during the summer.
Bring your own towel, beach chairs, and swim gear.
There are restrooms, including a changing room, and a concession stand.
Not warm enough to swim in the 510? Check out the 925 – it's usually warmer in Antioch where Contra Loma Lagoon offers a similar swimming hole.