The aggressive animal went after the woman and her 8-year-old grandson while they were swimming.
Tabitha Moser, the woman's daughter, was watching when the four-foot otter went after her son with its razor-sharp claws.
She saw the animal near the boy's head and heard him scream. The grandmother was closer to the boy, so she intervened.
"It followed us all the way up the river. It was biting her as I was dragging her out of the river," Moser told KING.
After the attack, the boy was taken to the hospital and his grandmother underwent eye surgery.
"I can't begin to go into the mind of this animal and tell you why it did what it did," Ruth Milner, a wildlife biologist at the Department of Fish & Wildlife, told The Everett Herald. "It could have felt threatened by the human activity in the area. Normally otters are fairly calm around people. They hang around boat docks and that sort of thing."
Otters can weigh up to 30 pounds so the damage they can inflict is no small matter.
"Animals have a fear mechanism and when it's triggered they can become unpredictable," Milner said.