Romila Higgins, 41, and Indali Higgins, 5, arrived at the beach around 3:30 p.m. along with another women and three other children, according to San Mateo County sheriff's Lt. Ray Lunny.
Higgins, Indali and another family member were wading about ankle- or shin-deep in the water when they were suddenly pulled under the water and into the sea, Lunny said.
A witness heard them yelling for help and swam toward them with her boogie board to assist.
She used the boogie board to keep Higgins and Indali afloat, but the three were hit by a large wave that knocked them off and pushed the mother and daughter farther out to sea.
After the wave subsided, the boogie boarder found herself close to shore and was pulled from the water by a bystander, according to Lunny.
She had thought Indali was still with her as she reached the shore and had no idea how the child was jerked back out into the sea, Lunny said.
Emergency personnel received word of the incident around 4:20 p.m. and searched the beach and ocean area for the mother and child.
Higgins was located first and a Life Flight helicopter transported her to Stanford Hospital, according to Lunny.
The girl was found a quarter mile south of Montara State Beach, unconscious and not breathing. She was also taken to Stanford Hospital.
Both Higgins and her daughter were pronounced dead at the hospital, Lunny said.