Business casual..evening?

You're probably right. Of course, "business casual" is, itself, a somewhat more nebulous term than, say, "Black Tie." Anyway, in this context, I suspect business casual is simply a somewhat clumsy way of saying "You're not required to get all dressed up in a suit and tie."
Hey, a winter nighttime boat tour of Detroit itself takes a liberty or two with traditional rules, so if you feel like wearing tweeds, by all means do so.

Also, the "no tweed after 6" rule is really just one of those London guidelines, like "no brown in the City." I mean, if it's a rule you just happen to like, and want to follow, that's fine. But don't regard it as a terribly binding rule for a party in Detroit. I know that if I were attending an evening holiday party, which was nominally "business casual," and I saw someone wearing a fairly tasteful tweed jacket, I wouldn't think for an instant that he's a clueless barbarian. And I'm probably more attuned to the traditional attire rules than most.
That's probably how I'd dress. Since it's a social (albeit, business related) event, I might put on a festive bow tie. But that's just me.

You might want to consider wearing a v-neck sweater under that blazer. I'm thinking that Detroit gets sort of chilly on the water, at night, in December. And the beauty of the sweater is that if you're the only man present wearing a jacket (extremely unlikely, I should think), you could always remove the blazer, and still be wearing something over your shirt and tie.

Seriously though, the vast majority of the time at these holiday parties, all business casual means is that you needn't wear a suit and tie, but you should show up in something nicer than jeans and a t-shirt.
--
Michael

Consider a suit without a tie. I've worn a charcoal gray suit with a taupe/gray/white paisley shirt as business casual before. I like to think that the muted colors of the paisley and the charcoal gray suit with the gray in the shirt muted the paisley.

That said, I like the idea expressed above of wearing your blazer with a sweater (or sweater vest) underneath. Layering during a Detroit river cruise this time of year sounds like a dandy idea to me and the sweater gives you a chance to show some more creativity.

Not sure what to advise, but always nice to meet a fellow Detroiter

I like the bowtie idea for this occasion, and I'm not usually a bowtie guy. A nice silk knit might do the trick as well

General consensus is that business casual, especially for a festive event, means next to nothing, like someone mentioned above, sort just thrown into say don't wear jeans and t shirt. Wear something warm and comfortable. I like the idea of flannel or tweed. Really, it's your call. Unless you know something more (like, is the set of crowd and people attending well to do people who probably would wear more "formal" things?)

fine knits or dressy sport shirts with dressy pants, ties not included.

sounds like a winner

Include a scarf like in Umbrella grazzo's avatar for an extra touch of louche-like elan!

FTFY, and thanks!

navy blazer with grey pants = security guard in my mind.

Your choice sounds okay.

Remember, it's just a holiday party; don't overthink things.