"Crying and then not responding. Not responding to like, what's the matter. It was very unusual for her," said David.
David, who was driving the boat, was barely going 5 miles per hour while Sierra and her mom were in a tube pulled by the boat 20 feet behind the motor during a parade with several other boats. She then sat in the back of the boat.
But when their daughter started acting up, they went to shore.
"She wasn't telling us anything," said Marnie.
"She fell over like a drunk. Three steps and into the yard," added her husband.
Turns out she was starving for oxygen, suffering from carbon monoxide poisoning. They first took her to the emergency room where Sierra's symptoms seemed to already be dissipating.
"By 15 minutes all of her symptoms had resolved, she was talking, she was fine but her CO levels were almost lethal for an adult," said Marnie.
Last year one of their neighbors suffered from carbon monoxide poisoning too while on a boat, they say. That's why Marnie says she asked the doctors to check for CO poisoning. She says her daughter's levels were very high, to the point doctors rushed her by ambulance to the Hennepin County Medical Center's hyperbaric chamber.
"There are particular places on a boat that are particularly dangerous for the accumulation of carbon monoxide," said Dr. Cheryl Adkinson who treated Sierra at HCMC.
She says this sort of thing happens more often than people think, but it's likely under reported because boaters blame the symptoms on heat stroke or alcohol use.
According to the CDC, more than 800 boating-related poisonings have been identified with more than 140 resulting in death. One in four of these poisonings are attributed to generator exhaust alone.
"You can get enough of cumulated carbon monoxide in that area to make someone seriously ill or even kill them," said Adkinson.
It's why the Schuster family bought a portable CO meter for their boat. When KARE 11 went for a ride, it was detecting high readings in the back of the boat within only a few seconds on the lake. The back of the boat can often times be the worst location because it's the lowest point and close to the motor.
"This is where we would sit and watch people surf," she said. "Scary. Who knew?"
"You feel like you're not a good advocate for your child. You do everything to keep them safe."
And that's why the Schuster's want others to remember their story, even if they may want to forget it.
"The good part about this and us telling this story, we have a happy ending," said David. "I guess our big part of sharing the story is how dangerous it can be," added Marnie.