Detective Patrick Michaud says both boats were able to return to a dock at Magnuson Park, north of the Highway 520 floating bridge.
Michaud says Harbor Patrol officers are investigating. They think the collision may have been a result of choppy water conditions and sun reflecting off the water. Alcohol is not believed to be a factor.
A teacher was killed and two other people were injured last month when a motorboat collided with a sailboat on the lake.