A woman whose husband and eight-year-old daughter were killed in a speedboat accident has described her desperate efforts to save her children as the out-of-control boat careered towards them.

Victoria Milligan, her husband, Nick, a senior Sky media executive, and four young children were thrown from the boat off Cornwall in May last year. Milligan says she was struggling to hold on to her four-year-old son and swim away from the boat’s propeller when she heard her 12-year-old daughter, Amber, crying: “Daddy’s dead, Daddy’s dead.”

“Lying in the water, dipping into unconsciousness having lost a lot of blood, I was figuring out how I was going to look after the children on my own,” Milligan, 40, wrote in the Sunday Times Magazine. “I would put the house on the market, buy a smaller one … It still surprises me how strong that survival instinct is.”

Milligan’s lower left leg was partially severed by the propeller and later amputated. Her husband and daughter Emily were killed.

On the eve of the inquest into the tragedy in Truro on Monday, Milligan described how the accident changed the family.

“Long-term plans for our future have very suddenly been taken away,” she said, adding that the grief could still take her by surprise. “Of course, some things are always going to floor you like when Olivia, my 11-year-old, asked me last week, in a very matter-of-fact way, ‘Mummy, who is going to walk me down the aisle now?’

“Or when I’m at the supermarket checkout and find I have put peanut butter in the trolley. It was Emily’s favourite and she’s the only one that ever ate it.”

The accident happened during a boat trip in the Camel estuary near Padstow in May last year. After a lunch of fish and chips on the boat, they went round the estuary. Milligan was concerned about the low tide and suggested returning to Rock, where they were staying. Her husband, Nick, took off the kill cord – a safety device worn by the driver that switches off the engine if pulled – to retrieve his sunglasses at the back of the boat.

Increasingly worried about the tide, she had started the engine by the time he returned to her side. Without reattaching the kill cord, her husband reached across her to take the wheel.

“At the same time as pulling the steering wheel down hard to the right, he pushed the throttle up to its maximum, causing the boat to go into a steep turn and we all found ourselves flung into the water,” Milligan wrote. “I can only presume that he was helping me to make the turn and slipped.”

The parents and all four children were flung into the water. Three canoeists rushed to the scene, performed first aid and called 999. Soon after, a watersports instructor jumped on to the boat from another speedboat and stopped the engine. Later, in hospital, she was told of her daughter’s death. “I knew that Nicko was dead as Amber had been screaming it in the water, but not my beautiful Emily,” she wrote. “It couldn’t possibly be real. I just remember feeling completely numb.”

Milligan later lost her lower left leg but Kit’s leg was saved after 12 operations.

Although the family live in Wandsworth, south-west London, Nick and Emily are buried at St Enodoc church in Cornwall, near the family’s holiday home.

Milligan, who has helped to raise £750,000 for Child Bereavement UK, the Royal National Lifeboat Institution and the Cornwall Air Ambulance, has returned to work as a personal trainer and is writing a book about the tragedy and its aftermath. She wants it to serve as a practical handbook on grief. “Nobody knows what to do or say when someone is bereaved and I want this to help people from personal experience.”

She urges others to value what they have and make the most of every day they have with their families. She also wants to raise awareness about the importance of safety on boats. “I can never change what has happened to my family but I can urge other people who use power boats to always wear a kill cord and to do the relevant training.

“My family has been ripped apart and two wonderful and special people taken away from me. I used to resent the bedtime reading, now I would give anything to read Emily one more story and kiss her goodnight one more time.”

• The headline of this article was amended on Sunday 10 November 2014.