Police said the dead woman was one of three people thrown from the 21-foot sailboat Wednesday night. The other two were reported to be in serious but stable condition.
Investigators were trying to determine if either of the boat operators were drunk.
The boats were about half a mile from the shore when they struck each other at about 10:30 p.m.
"We were asleep on our roof and we heard a loud bang and we heard people crying for help, and then many, many sirens," Betsy Hale, who lives on the lakeshore, told KOMO-TV.
Both boats were damaged by the collision but were still afloat. Police said there were four people on the powerboat, and one, a woman, had minor injuries, while the sailboat was carrying seven people.