I just posted this as a Q&A:
I’m English, with a sort of sloppy version of received pronunciation, so I say something very close to “boy”.
Most Americans pronounce it closer to “boo-ey” (which I discovered as a boy, and not a buoy, listening to the LP of A Funny Thing Happened On The Way To the Forum).
Unless they are advertising soap. http://www.youtube.com/watch?v=GzCe1kxlZm4
Which also does not explain why Americans pronounce “buoyant” as “boyant” and not “boo-ey-ant”
…………

And then I thought, I wonder what Frankie Howerd sang on the original 1963 London version of A Funny Thing Happened On The Way To The Forum when he got to the line about the “bong of the bell of the booey in the bay”, so I went down to the basement and retrieved the CD and found out.
I learned that line is the same, but that ”Past the buoy (boo-ey) and through the bay” needed an extra syllable for the English audiences and singer. I imagine that Sondheim rewrote the line himself…
And I am putting this song up here for the twelve Stephen Sondheim/Frankie Howerd/pronounciation nerds on the whole of the internets who will be as interested in the answer as I was.