Maffei, 43, allegedly abducted his daughter Brooklynn, 3, and son Devin, 2, from their mother's home in South San Francisco Tuesday and then stole a 40-foot sailboat and sailed away, according to a South San Francisco Police Department press release.
A fisherman told police he spotted Maffei and Brooklynn on the boat early Friday morning, the San Mateo County Times reported.
The U.S. Coast Guard and other law-enforcement officials were searching for the boat, but had not made contact with the sailboat as of 11 a.m. PDT (2 p.m. EDT), police told the Times.
Authorities will try to communicate with the man believed to be Maffei once they are able to locate the boat.
"The Coast Guard is trained for that; we will defer to the Coast Guard for tactics on that," South San Francisco Police Sgt. Bruce McPhillips told the paper.
The stolen 44-foot boat, named "Unleashed," may be headed toward Monterey, Calif., or Mexico, according to the police department's press release, which indicated that Maffei "has expressed interest in the past of sailing" to those locations.
Anyone with information on the sailboat, Brooklynn, Devin, or Maffei is urged to call the South San Francisco Police Department at 650-877-8900.
Maffei used a rental car when he allegedly abducted his children, according to police. The vehicle was found abandoned Thursday in the parking lot of the Alameda Marina.
South San Francisco police said their investigation of the abduction uncovered that Maffei, with his children, visited the owner of the boat and asked about purchasing the vessel two weeks before it was stolen.
Authorities said an "extensive air and ground search was conducted" on Thursday, "but the vessel was not located."
Brooklynn was last seen wearing a white, sleeveless dress with blue and green designs, while Devin was last seen wearing a short-sleeve Spongebob Squarepants shirt and a diaper, police said.