Don't do this. The last thing you want to be in is water doing breath work. I did a week long course with Wim Hof before he got popular, they were very adamant that you never do breath work in water. This is just ignorant and dangerous advice. I've been doing breath techniques for over 5 years now and would never do them in water even though I'm well practiced.

I’m considering taking a WHM afternoon class, but have been questioning my expectations. For a noob, do you think a couple hours in a workshop would be valuable? Or am I being suckered by charletons?

My guess is you've read "What Doesn't Kill Us" and maybe Scott Carney's other book(s). I did a week long course in Poland - I can't say enough good about my experience, I'd do it again (and am planning on it).

I've not done weekend / afternoon type workshops. My guess is this is going to give you a bit more than what you've done on your own. Group breath work has a whole different flare to it (in a really good way, and I'm not an overly social person). So, it may just be an upsell into the longer courses, but I will tell you learning breath work in a group setting has an energy you don't get on your own. Plus you may pick up a few trips to help you hone in on control. Just my $0.02.

Expanding on why this is dangerous, doing these breathing exercises in water is bad because the drive to breath (excess co2 in blood) can be postponed via hyperventilating beforehand, but this means you can still pass out from a lack of oxygen in the blood. If this happens while you are in a bathtub, you will drown.

I discovered this as a kid simply trying to relax in the tub. Could sometimes leave my airways open but stop actually breathing for long periods of time. Very relaxed state. Probably the closest thing to an altered state of consciousness I’ve ever had. Haven’t done it in decades.

Yeah dude, better avoid doing these kind of things in water

Also don't try while driving.