You are right, but please note that the verb in the plural should also indicate that it has an object. The correct form is 'köszönjük'. Making a distinction is important, for example: 'köszönök neked' means 'I say hello to you', while 'köszönöm neked (hogy segítesz)' means 'I thank you (for helping me)'.
I'm no expert in Hungarian, but to me it looks like köszönöm is actually already the first person singular conjugation of the verb "to thank," so it really means "I thank." In English, we would say "(I) thank you for the water," so "you" is in the accusative case, and "water" is in a prepositional phrase. In Hungarian, it looks like they put the thing being thanked for in the accusative. Therefore, "I thank the water" actually translates to "I thank (you) FOR the water."
"Vizet" is the accusative form for water, so this sentence would translate appropriately.
I may be wrong here though, so if someone knows better, do please tell me :)
Köszönöm is actually the first person singular conjugation of the verb "to thank" (köszönni). In English "I thank you for the water", you is actually in the dative case (as it would be in Hungarian if we wrote it in full, "Köszönöm neked a vizet", neked being the dative form of te. And yes, the thing you are thanking for in English is in the accusative case in Hungarian.
Hmm, I don't know how you can claim that the "you" in "thank you" in English is in the dative case when we never morphologically distinguish accusative from dative. Yes, it may have been dative in Old English and it's dative in German, but it's a meaningless question in English.
Hmm, it sounds like your assuming that the thing you are thankful for is somehow still more of a direct object than the person who is thanked, even though in English the thing is introduced by a preposition and not an object at all and the only possible object role is the person. Is your reasoning because when there are two objects, most sentences in most (nom-acc-dat) languages will end up with a person in dative and an inanimate thing in accusative?
In English, when there is only one possible object role, it's really a moot point as to whether the object is accusative or dative. We can see a difference when there are two object roles, because the dative object must precede the accusative (or else be replaced by a prepositional phrase).
What would you say about the phrase "to trust somebody with something"? In German, the phrase is "jemandem [DAT] etwas [ACC] anvertrauen". In English, the structure is like "thank" ... a person as the only object role and a thing (or a person, maybe a baby, for example) introduced by a preposition?
What about "to cast someone in a play/film"? Would you say that "someone" is dative and the play or film is a more logical accusative role that English happens to have as a prepositional phrase?
(I'm not trying to be confrontational, I'm just interested by your views.)
I was about to write a response to this, but then I had the idea of checking other related languages to see since English doesn't usually inflect most things for case. Sure enough, here's the full sentence in German:
Ich danke dir für das Wasser.
NOM / DAT / ACC
So "you" would be in the dative, you're right. You can also see more clearly that Wasser is in the accusative if you use a masculine noun. It seems like the full phrase would be "I give thanks to you for the water."
Für always takes the accusative in German, though, so I'm not too sure about some of that.
And if you wanted to get technical, you is neither dative nor accusative because those two cases have merged in English and formed the oblique.
I agree with akidaki, either "a víznek köszönhetően" (or even "a víznek hála"; not literal translation, but the meaning is the same in this context), or it can also mean "köszönet a víznek" (this would be the case if, for example, you would like write a poem or something and name it "köszönet a víznek" which would be about expressing your thankfulness to the water :D)
For me, it's the concept of "implied for" which sounds funny. :) By Hungarian logic, "thanking" is not a standalone action that you append a reason or goal to, it's not a social engineering skill. Your thankfulness is directed to what you got and it's less relevant towards whom you express it.