The family had spent their summer holidays at Genevieve's maternal grandparents' - Martin and Margaret Genet - bach for years.
Curtis has undertaken restorative justice with the family, but her parents said yesterday his sentence had "let our daughter, us and the community down". The maximum penalty for the charge is a $10,000 fine or 12 months' imprisonment.
"People need to be held more accountable when they take a life," the Lewises said in a statement.
"We have always tried to keep a high standard of safety in and on the water for our family and others and there was nothing different about that on the day Genevieve was killed.
"Being utterly powerless to prevent her death has been one of the most difficult things to come to terms with.
"Nothing can bring our daughter back but something positive must come out of this tragedy. But we don't want anyone else to die or be injured or fearful because of irresponsible use of boats.
"We want people to understand the responsibilities they have when driving a boat or a jet ski, learn and follow the rules, and take greater care."
By law, boats are required to go no faster than five knots or 9km/h, within 200m of the shore or 50m from another boat or swimmer.
Lake Taupo harbourmaster Phillip King said winter was the perfect time for boaties to be studying the rules for the water or attending a harbourmaster skipper course.
"It was a tragic event and I think the boating public need to take education seriously and be aware of the consequences of not knowing what's going around them when on the water. The potential for a tragedy to occur is there at all times."
Michael Mercer, principal of Masterton's Hadlow School, which Genevieve attended, said a photo of the 9-year-old had been erected at the school to "remind us each day that she was here and she was one of us".
"A court case is a rather sad end for it because we're remembering Genevieve as the little girl that she was when she was here.
"I don't think there's anything that you can do or say that's going to be enough for the family because I know the grief that they're going through."