That was quick! I guess crowdsourcing really does work. Thanks, reader Maya H., of Hyde Park, for identifying Eva Morrison.
According to this vintage postcard, Morrison is "the only woman in the world to make the Boston Light swim and return of 3 miles."
A visit to Flickr reveals that another user has offered more info about Morrison:
Eva Morrison of Roxbury MA was a prominent distance swimmer in the 1920s, best remembered for making a record-breaking time in the Boston Light Swim, and training to swim the English Channel at the same time as Gertrude Ederle did. She served on the initial Board of Governors for the International Professional Swimmers' Association when it formed in 1927. Morrison was inducted into the International Marathon Swimming Hall of Fame in 1973.
Finally, I discovered a 2d image of Morrison in the LoC photoset; it wasn't tagged "Boston," which is why I didn't find it the first time. (I've added the tag.) Here it is:
The author is solely responsible for the content.