51-year-old Nicholas Milligan, 51, a senior executive at BskyB, was named as the father who died in the horrific accident in Cornwall.
Charlie Toogood from Camel Ski School, was praised by coastguards who said his "incredible seamanship and bravery" prevented further injuries.
The family-of-six were thrown from the speedboat in Padstow harbour yesterday afternoon, before the boat veered into them, killing Nicholas Milligan, 51, the managing director of Sky Media and his eight-year-old daughter as well as causing to others.
Witnesses watched in horror as the "sea turned red with blood". Mr Toogood pulled alongside the circling boat and leaped across, managing to get the boat back under control by shutting down the engine.
Matt Pavitt, of the North Cornwall Coastguard, told the BBC: "Thanks to some incredible seamanship and bravery from some locals, they managed to get the boat under control and stop it from causing any further damage or harm.
"From there we were able to locate, recover the various people with injuries working with our colleagues from the ambulance service, Navy and RAF helicopters, two lifeboats and three coastguard teams.
"In a major joint operation, four of those people were flown to Derriford Hospital in Plymouth with significant injuries."
Mr Pavitt added: "It is, as you can imagine, a very large boat. This boat is eight metres long with a very, very powerful engine on the back. It is going round and round in circles and one of the local boatmen, we understand, was able to get alongside it and somebody actually jumped from one boat to the other to bring the boat under control."
He said the cause of the accident wasn't known yet. "Reports are that the boat was seen to veer over to one side, causing all six people to end up in the water and then the boat is known to have circled a number of times hitting some of the people in the water.
"That is as much as we know at the moment."
Four other me mbers of the same family were flown to hospital with "life-changing injuries" after they were flung overboard and hit by the out-of-control boat.
Some of the victims may have been hit by the propeller
Det Supt James Colwell said: "This is a tragic incident. What we know at the moment in that we have two fatalities, a 51-year-old adult male and an eight-year-old female. We also know this was an incident that happened in the middle of the river involving members of the same family.
Simon Lewins, from Wadebridge, was just 100 yards away when the accident happened just before 4pm on Sunday.
He told BBC 5 Live: "We saw a boat coming up the estuary and it seemed to take a right turn. As it took the turn it flipped and the people in the boat went out in the water and then the boat went round in circles and it seemed to come back into the group on three or four occasions.
"We heard screaming and saw seagulls coming into the water, it wasn't good.
"As this happens the boat was taking ever-decreasing circles with nobody on it because they were all in the water and it was slowing and it kept going for 10, 15 minutes.
"We watched this in horror and it lasted quite a while. As that happened the in-shore lifeboat came out and a few other boats came to look at what was happening and the local speedboats from out of Padstow harbour came to attend.
"I must admit the inshore lifeboat did a fantastic job because it came in towards the boat that was going around in circles and a guy from the lifeboat got into the boat and arrested its movement quite quickly."
Another witness, Martin Jenkins, 63, added: "People were lying on the floor in a lot of discomfort. They were airlifted away on stretchers.
"Everyone was ushered away so we couldn't see any more.
"It was probably for our own good because it looked terrible. The water was filled with blood like a horror film."