"Who is drinking the water?"
Translation:Hvem drikker vannet?
That depends on the grammatical gender of the noun. In this case 'vann' is a neuter noun, so you should use the neuter inflection. et vann - vannet.
-en at the end is used for masculine inflection (which feminine nouns may follow). en sko - skoen.
The grammatical gender has to be memorized.