Mr Watkins said it had been a long year of healing, during which time the family had moved to Hastings for a fresh start.
"It's devastating. Terrible. It's just ruined me and my wife. It wrecked our marriage at the start. We're back together now but the pain of losing a child is just unbearable. It's taken us until now to ... [begin to] move on."
Back in Huntly during the Christmas holidays, the family including brothers Tyrone, 22, Kayhne, 16, and Tamatoa, 10 months, visited Zharian's grave behind Taupiri Mountain every day.
Mr Watkins said Maori children in particular were risk-takers and the family struggled at first with apportioning blame.
"There's no one to blame. My son made the decision to go for a swim."
He encouraged people to enjoy the water but urged them to remain vigilant.