> 47% of Americans cannot meet a $400 emergency expense. You deny their poverty?
Yes. You yourself posted: "About 15% of the US population (50 million people) are at or below the Federal poverty rate." You refuted your case.
> small cohort
I've seen these middle class people everywhere I've worked. They're not a "small" cohort. In fact, most of the people I've worked with fell into this category. As far as I could tell, I was the only engineer in the office building who did not deposit the paycheck the same day. I know this because others expressed shock that I didn't.
> it still knocks a rather major hole in a foundational premise of free-market economic theory
No, it doesn't. These people chose to behave this way. It's their right to. Nobody made them buy the boat. One of these boating people would sell his boat when he needed cash, and when he had cash, he'd blow it all on another boat, or a new car, or an RV, or whatever caught his fancy. Free market theory posits that people have a free choice. Not that what they do makes fiscal sense to anyone else.
> It's a poor basis on which to make assessments of national wealth, poverty, and/or inequality.
How much cash is in your bank account is a very, very bad assessment of wealth, poverty, and/or inequality. But that's what you're putting forward.
For another example, I have next to $0 in my checking account, and don't have a savings account. But I'm not poor, even though you'd classify me as poor. I keep it all invested. I only use cash as a transfer mechanism.
P.S. I actually did have a boat once (!) and me and my friends had a couple marvelous summers waterskiing on Lake Washington.