Sorry to make you homesick! That was indeed Williamsport.
I have not biked as much of the canal as you did, but as a child and teenager I canoed every part of the river from Knoxville (a little bit downstream from Harper’s Ferry), down to Swain’s (just upstream from Great Falls)- many times. We used to camp on the islands in the river - by “camping” I mean we spread a tarp out on the ground and passed the night. By the time I was in high school, my father had no problems dropping me and a friend in at Brunswick or somewhere and saying “see you at Swains in a couple of days!”.
He used to drop us off on the Appalachian Trail as well, and pick us up a couple of days later at a predetermined rendez-vous. All this being many years before cell phones of course.
This last November I did a short overnight bike trip from my house in Rockville to the hiker-biker near Edward’s Ferry and back, to test out my budget setup, and hopefully this year I can find the time to take a couple of longer trips.
You may also be interested and possibly sad to hear that White’s Ferry has finally ceased operations.
https://en.wikipedia.org/wiki/White's_Ferry
(Funny, I was unaware of the somewhat, uh, dubious Confederate associations of the place until I just now read that, despite having read a book about Ball’s Bluff many years ago.)