The non-injury mishap was reported shortly after noon about 10 miles southwest of Point Loma, according to the U.S. Coast Guard.
Cici Sayer, captain of the boat Offshore Blue Adventures, told Fox5 she took a man out to take pictures of blue whales. The pair was about 150 yards away from the blue whale they were tracking, which she considered adequate distance from the animal, when a second whale surfaced next to their boat, dumping them into the sea, Sayer said.
Passengers aboard another whale watching boat saw the blue whale tip the boat over and helped get the victims out of the water, USCG Petty Officer Connie Terrell said.
A commercial salvage vessel then was called out to right the damaged boat and tow it Shelter Island.