"You have an apple, bread and water."
Translation:Macie jabłko, chleb i wodę.
"Ty masz" is singular and "Wy macie" is plural. Both translates to "You have" because English has lost distinction between second person plural and singular and uses the same pronoun in both cases.
I'm always getting way these wrong on the first try :( How do they want us to determine from the English version if they want singular or plural in Polish?
We don't want you to do it. You choose yourself (and I'd advise to practice both of them).
In 99% of the cases, there's absolutely nothing in the sentence to suggest either singular or plural. Both are equally correct. It's your choice.
"mieć" (to have) takes the Accusative case, which is "wodę".
Neuter nouns (like "jabłko") have Accusative the same as Nominative (the basic, dictionary form), and masculine inanimate nouns (like "chleb") also have those cases identical. But still you should be aware what case is this.