German Translation
Dieses Wasser ist nicht trinkbar.
This is the passive voice, right?
As I understand this sentence, literally it says:
“This water should not (must not) drink itself.”
As discussed here, this construct of a noun referring to itself/oneself is used to convey the passive voice.
So whereas the German/English originals both say “This water isn’t drinkable”, a translation closer to the Spanish sentence would go more like “This water should not (must not) be drunk.”, am I right? (In German, “Dieses Wasser sollte/darf nicht getrunken werden.”)
Would someone please confirm or correct this thought? I’m a beginner just starting to get the hang of this. Thanks in advance. 
I’d say this is a case of the impersonal “se”:
Dieses Wasser darf/sollte man nicht trinken.
But as we can see there’s multiple ways to convey the idea:
Dieses Wasser ist nicht trinkbar.
Dieses Wasser darf/sollte man nicht trinken.
Dieses Wasser darf/sollte nicht getrunken werden.
I’m unsure about this. Any sentence that uses the “impersonal se” mustn’t have a subject. But here we have a subject (“Who or what mustn’t be drunk?” “The water.”). The agua is either “doing the action to itself” or being acted upon, depending on how you view it (active or passive voice). In either case, the agua is the subject of the sentence, which is not allowed with the “impersonal se”. So, ruling out the “impersonal se”, it must be the passive voice, I think.
Thanks for your feedback, I appreciate it. 
Per the definition in the linked article, you are right.
Practically, if you treat it like german “man”, it functions the same whether you call it passive or impersonal, and you can interpret “Este agua” as the object of “tomar”, and it works the same with or without an object, or whether you have an expression of time or location instead of an object (such as “Se come bien en la playa”). The only thing to watch out for is the change of the verb to plural with plural “objects”.
Of course, literally/grammatically “agua” is the subject of a reflexive construction.
There’s ways to create a reflexive analogy in german (although the meaning is not quite the same):
Dieses Wasser lässt sich nicht trinken.
Or another example:
Esta puerta no se puede abrir. - Diese Tür lässt sich nicht öffnen. (= “kann man nicht öffnen”)
For practical purposes, especially when coming from german which has “man”, I think it’s helpful to think of the “ser + verb + por” construction as the “real passive” as we know it from other languages, and the “se” variants as the “impersonal” form that corresponds very much to german “man”, even if it’s technically not correct, and grammar guides define it differently - simply because the analogy fits too well and only has that little gotcha with the plural.
It’s a bit like thinking of electric current flowing through a wire when in reality it’s electromagnetic fields around the wire: Electricity does not flow through wires! - Energy One
It’s not correct, but good enough for practical purposes and it makes your life easier.
Edit:
I mean, calling it a “passive” is already a simplification when it’s really a reflexive under the hood…
I found this site in german which explains it quite extensively: Passiv in der spanischen Grammatik
It turns out that the distinction between “pasiva refleja” and “impersonal refleja” is not whether there is a noun or not, but whether the “object” is a person or not. When it’s a person, the preposition “a” comes into play, and the verb stays in singular:
Se ha contactado a los testigos.
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Thanks for the answer! There’s a lot to process, I’ll get back to you once I combed through it.
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