The Yakima County sheriff's office says the 29-year-old woman and 14-year-old boy who drowned together in Rimrock Lake didn't know each other.
A number of people were at the lake about 25 miles west of Yakima Saturday when the woman attempted to retrieve a floatation device that had drifted into the lake and the boy offered to help.
Witnesses said the boy panicked and struggled with the woman, exhausting them both.
Witnesses pulled her from the water, but the Yakima woman could not be revived.
Sheriff's divers found the teen's body two hours later, about 25 feet from shore in 10 feet of water. He also was declared dead at the scene.