An elderly woman and her two dogs died yesterday afternoon after the car they were in plunged into the causeway at the King Harry Ferry slip.
The driver, her husband, was not in the car as it bumped against another car and then entered the water on the Roseland side at about 4pm.
The ferry was at the opposite side of the causeway at the time.
It is unknown whether the couple, both in their seventies, are local to Cornwall.
Police, fire and coastguard were scrambled, aided by RNAS Culdrose and police divers.
Local mussel diver, Matt Vernon, carried out numerous attempts to rescue the woman.
He told the West Briton: "The car was at least 25ft underwater. It was impossible to try and smash a window.
"I got round to the front of the car and found the passenger window was wide open and the car was filled with water.
Diving without oxygen, Mr Vernon had to resurface to take a breath before going back down to the car.
He said the water was very murky and visibility extremely poor.
He attempted to attach a grappling hook to the car but it was not strong enough to take the weight.
Both Falmouth's All Weather and Inshore Lifeboats were on the scene by this time aiding the rescue attempt.
A line was eventually secured to the car by commercial divers who were brought to the scene by the MTS Valonia. The car was totally submerged for at least 30 minutes.
Shortly after 6pm the Fire Brigade managed to pull the car back onto the slipway and the body of the missing women was recovered from the car.
The channel, called the Carrick Roads, stretches from Falmouth to Truro and is famous for its size and depth.
It has the third largest natural harbour in the world at its mouth, and in parts depths reach up to 34 meters.
The King Harry Ferry released the following statement: "Our thoughts are with the friends and family of everyone involved in yesterday's tragedy. The ferry was not involved in the incident and our normal service has resumed."
The ferry connects St Mawes and the Roseland Peninsula with Feock, Truro and Falmouth and is one of only five chain ferries in England.
See a map of where the incident happened:
View Woman dies in tragedy at King Harry Ferry site in a larger map