"She has water."
Translation:Sie hat Wasser.
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einen is masculine accusative singular, but Wasser is not masculine but neuter.
Also, Wasser is normally uncountable, so we don't use it with the indefinite article -- just as we say "She has water" in English and not "She has a water", so in German we would say Sie hat Wasser and not Sie hat ein Wasser.
For the same reason that you can't just say "Why doesn't you just say doesn't you?" or "He am my friend but you is my friend too".
You have to pick the right verb form that matches the subject.
Here, the subject is sie and the appropriate verb form for that is hat.
hast would be used for the subject du.