Senior Sergeant Tony Jeurissen said the young girl "had horrendous injuries".
The Herald understands Genevieve, who liked riding motorbikes on her family farm in Masterton in the Wairarapa, suffered severe lacerations and cuts.
The boat driver stopped to help the family and Genevieve was taken to shore, but she died soon after the accident, despite the efforts of St John Ambulance officers.
Genevieve's maternal grandparents, Martin and Margaret Genet, were among family members who gathered at Waipahihi Bay yesterday afternoon as a Ngati Tuwharetoa kaumatua placed a rahui (prohibition) over a small area of the bay, temporarily banning all activities there.
Mr Genet, 77, of Dannevirke, said the family had had a wonderful Christmas and New Year holiday when this "horrible, horrible tragedy" occurred.
"Her parents are devastated, of course, we are all devastated, it's just awful, we can't believe it's happened," he said.
His granddaughter held a special place in his heart, he said.
He often called her "Scout" after the character in the Harper Lee novel To Kill a Mockingbird.
"She had a very similar personality to Scout, she was a wonderful little girl...she was very popular at her school," he said.
Mrs Genet said it was unclear where Genevieve's funeral would be held, although she understood her body was being taken back to Palmerston North, possibly today.
Detective Senior Sergeant Todd Pearce said it was not yet known whether criminal charges would be laid.
The boat that hit Genevieve had been seized for a marine engineer's examination.
