The two had been fighting to divert floodwaters from their Eskdale home for three consecutive days.
But weekend rainfall of more than 200mm, a dysfunctional drain across the road and a three-stage agricultural dam overflowing on the hill above them meant there was little more they could do.
"My father, he came out to me and he's like, 'Righto, let's get the boogie boards,'" Mr McKoy told AAP.
The young carpenter warned his dad the water level wasn't high enough to ride the drain, but they didn't have to wait long.
"Within 10 minutes, it was just fully gushing out and then all the neighbours are joining in, everyone's getting on the kneeboards down the drain. It was so much fun."
The next day, after the weather had cleared and floodwaters had largely drained into Little Snowy Creek below them, Harry and his father assessed the damage and got to work.
"We had so much mud, like all our driveway washed away and just all went into our shed," Mr McKoy said.
Two days later, Mr McKoy said they were starting to get on top of the cleanup, but that it might only be a taste of what's to come.
"Now we know which way the water's going to come from, because they reckon there's more rain on the way, and it's got nowhere to go," he said.
Further downstream in Tallandoon, cattle farmer Beate Barnes has struggled to get to work after her house became surrounded by floodwaters.
"We have a moat," Ms Barnes told AAP.
"If I have to go somewhere or go to work, I would put my gumboots on, wade through to the car and then change."
"But my kids love it, they're kayaking."
Ms Barnes said despite dry conditions in recent days, floodwaters were still rising around them as waters came off the hills and into the rivers and creeks nearby.
"We've got seven centimetres left before it's in the house," she said.
Ms Barnes said she had not yet lost any stock, but feed was becoming an issue and they had not been able to create any silage.
"We don't just not have grazing area, we also don't have anything preserved for when we need it most in winter."
The Victorian State Emergency Service responded to 124 calls for assistance in the 24 hours to midday on Wednesday, including one inland water rescue near Russells Bridge at Golden Plains.
Lilydale's SES unit in Melbourne's east was the busiest, responding to 17 calls for assistance.
The SES continues to warn Victorians against entering floodwater, which can contain chemical contaminants, raw sewerage and dangerous debris.
The Environmental Protection Authority continues to test floodwaters and has said levels of E.coli, trace metals, volatile organic compounds, pesticides and other contaminants are generally below levels of concern.
Detected levels for perfluoro-octanesulfonic acid (PFOS) are also consistent to levels detected in agricultural sites in previous authority studies.
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