The incident happened near Avila Beach, located along the state's central coastline.
"I still have an adrenaline rush!" Julie McSorley told FOX26.
McSorley and her kayaking partner, Liz Cottriel, were off the coast of Avila Beach, watching the whales. The two thought they were at a safe distance, about 30 feet away. They quickly learned they were not.
"I saw the big pool of fish, the big bait ball come up out of the water. I saw the whale come up," McSorley told FOX26 on Monday. "I thought, 'Oh no! It's too close.'"
Video from onlookers shows the moment the huge mammal breaches the water’s surface and knocks over the kayak with the two inside.
Both women went into the water. Cottriel told FOX26 she thought the whale was "gonna land on me."
While video appears to show the whale had the kayak in its mouth, McSorley claimed they were just overturned and thrown underwater.
After surfacing, the two were rescued by other paddleboarders and kayakers.
"We got back to the car, I was shaking my shirt and a bunch of fish came out of my shirt," Cottriel said.
Humpback whales are known to be active near the California coast year-round, and officials advise people to keep a football field’s distance between themselves and whales when possible to avoid situations such as a capsize.
Harbor officials told KSBY-TV both women are lucky to be alive.