Or it could simply be a turbine rotor - some of those are pretty wicked-looking in the universe we live in.
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Ah, well that works, too.
To say it short and sweet, it was realistic enough to me that my mind immediately rationalized it with similar existing technology, and it didn't take me out of the movie.
Exactly, and the transparent (aluminum?) rotor housing was nothing more than a Hollywood conceit, together with Kirk's quick glances and "No, no, no!" conveying to the non-Trekkies (and the non-hydraulic engineers) in the audience that "Oh, dear - Scotty could be in some serious trouble, here!"
The thing is, water is a reactant. It reacts with all sorts of things. Why not just label it "Fresh Water"?
It may not be fresh water. It could have been heavy water.
Even if not, water is not always a reactant; in many reactions where water is present, it is--for the purposes of that reaction--an inert compound, serving perhaps only to moderate the rate of reaction without being chemically part of it. And if it's part of a circulating system in which such reactions take place, "inert reactant" makes more sense than "fresh water," which would be part of a different and completely isolated system, denoted perhaps by blue flanges rather than the "inert reactant" orange.