My example is definitely not for immersion. Of course you're right about stucco; I've just upgraded it with a finish coat appropriate for a good shower, rib lath to keep it from cracking for as long as possible, and StegoWrap to deal with the water that gets through the plaster. You could get rid of the pool mix and make it more like concrete than a fine finish, and you could get rid of the rib lath and StegoWrap if you're going over CMU (in which case I would use Hydroban).
I am speaking from personal, practical experience. I've done exactly all that, on showers and walls, mostly tadelakt, just a couple portland cement plasters, interior and exterior. Having done some, and having seen some failures in plaster showers, I would be cautious about doing them at all, and ultra-careful when I did.
I haven't done any in a couple years - the market for luxury plaster work disappeared for a while, and me standing in a shower, covered in white plaster, using a trowel to aggravate my carpal tunnel syndrome, isn't where I'm trying to take my business. But I do watch that market, and I'm not aware that anything has changed the plaster business in the last few years the way the membranes changed the tile business.
Sorry to go on so long about it. I suppose I miss that business a little bit; it had its own moments of craft if not of art.
Edit: I'm also trying not just to be old-school about it. When it comes to tile, I'm 100% nu-skool, all about the orange stuff.
Edit again: I doubt I ever broke even on a plaster shower. Much too much work.
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CSFO's example is for containing water, that's 100% constant immersion. I'd imagine a stucco app would work here because, I mean, there are concrete showers, porous stone showers, etc.