Yes, it is totally impolite in Hungarian to "want" - "akar". In US English it is okay I guess, in many situations. And, for example, in Turkish, to "want" is the actual correct way of asking for something, it is not rude at all.
But in Hungarian, one has to "ask for" stuff. "Kérni".
i am not sure how you got that suggestion, according to the top of this discussion it should be "I do not want any water."
The "No water, thanks" could work, though that is rather "Nem kérek vizet, köszönöm". For duo's lesson I would just stick with the "I do not want any water", for RL, yours is good too.