The bathtub worked, but it was because of the clicking sound of the leak, not bubbles.
At some point right after a partial rotation, the bathroom was so quiet that I was able to hear a faint click, followed by another click maybe three seconds later, then another, etc.
To more isolate the click, I rotated the tire back into the water until the click stopped, then back out until I heard the click again. This pretty much isolated the relative position across the tread width. Then I rotated that portion of the tire to the topmost position and, finally, literally using a magnifying glass, I was able to locate the puncture, as shown in the photos below.
After that I plugged the hole using a plug-kit that I had purchased a few years ago to repair my daughter's tire.
I then pumped up the pressure to 35 psi and, using my grand daughter's very old blowing-bubble solution, carefully checked the plug for leaks. There were none, but I'll be checking tire pressure over the next few days and weeks to verify that I stopped the leak.
Now I don't have to worry about that tire going flat while in storage this upcoming winter. Great!
Thanks again for the advice.
(In the left-most photo I've pointed out the old plug and the tiny puncture, the latter of which is just below the right edge of the black tape, in the channel between treads. The middle photo has a zoomed-in view of the tiny puncture and, surrounding it, what looks to me like a round impression of a nail-head. The right-most photo shows the old plug and the new plug.)