You can’t take a ferry to Mercer Island, as it is in Lake Washington, not in Puget Sound.
The Hiram M. Chittenden Locks, built in 1911 and often nicknamed the Ballard Locks, provides a link for boats between the salt water of Puget Sound and the fresh water of the Ship Canal, which connects eastward to Lake Union and Lake Washington.
I checked the date of the last ferry service on Lake Washington. The ferry Leschi makes its last run, ending ferry service on Lake Washington on August 31, 1950.
However, water taxi—”mosquito fleet”—from the ferry terminal on Seattle’s waterfront to Mercer Island was do-able through the Hiram M. Chittenden Locks. As a girl who lived on Mercer Island, Stanley Ann was well aware of her transportation resources. She may have taken the bus across the Floating Bridge. But, the idea of taking a water taxi seems like a easier task. She would step from a bigger ferry to a smaller boat If she came from Vancouver BC.
She would have known the route very well because it also offered her a fun way to get from isolated Mercer Island to the “cool” University of Washington coffee shops along the “Ave” as a rebellious bohemian teenager (called “Fringies” at that time). A water taxi ride from Mercer Island to the University District on the shores of Lake Washington is a more direct route. I am sure she used this resource, rather than drive a car or go by bus across the Floating Bridge, to get to downtown Seattle as a girl.