Emergency services were called at 11:44 BST to reports a boat carrying a father, his teenage daughter and her two friends had capsized.
Torbay inshore and all-weather lifeboats were called and the four were taken to Torbay hospital.
The coastguard search and rescue teams from Berry Head and Torbay and the coastguard helicopter were also called.
Specialist divers are at the scene and are expected to remove the capsized boat from near the entrance to the harbour.
Mark Criddle, coxswain of Torbay's RNLI lifeboat, said the vessel was launched at 11:51 BST to reports of an overturned boat just outside Brixham harbour with four people in the water.
'We arrived on scene to find a 16ft speedboat upside down with a girl trapped in it," he said.
"We located the girl and brought her onboard the lifeboat where she was immediately given first aid by a doctor and paramedic who were members of the lifeboat crew."
He said the three other people who had been on the speedboat had been rescued by other craft in the area, including two jet skiers and a yacht.
'We took the girl back to shore where she was handed into the care of paramedics who took her to hospital. The three other people who had been aboard the overturned speedboat were also taken to hospital after being treated at the lifeboat station."
One person died and three others were taken to hospital after a boat capsized in Brixham harbour in south Devon.