I voted for "bow", but I agree that "bo" is more accurate to what I say (and what I think is the intended way).
I can just imagine talking to an English speaking Animal Crossing fan who's talking about "Bee-you" - I would never guess that they could mean "Beau"... but it's really cute, I like it as a nickname
Anyway, Beau is named Martin in my game since it's in German. Hehe.
When I was in Middle School we read a book for an assignment and the young woman in the story would refer to her boyfriend as her beau. The teacher told us it was pronounced bow and not Be-you. So I have always pronounced Beau as bow.
This probably doesn't make any sense to you English-speakers but "b??" is how I pronounce it. It should be close to how beau is pronounced in French and how you guys pronounce bow.
For the longest time I've pronounced it "byew" but I have a kid in one of my classes who has the name Beau so I'm beginning to pronounce it "bow" because that's how it's spose to be.
I've always pronounced it as "bow". Honestly, I thought everyone did. I haven't had a thought about pronouncing it any other way than that, until I saw this thread. And it seems it's actually a few that pronounces it "byew", hah.