"They didn't drink water"
How do we know it doesn't mean, "They aren't drinking water"?
if the verb is negative and past or future tense, it ends with '-a' if the stem verb ends with '-a' (which most do). If it's negative and present tense and ends with '-a', that changes to '-i' i.e. for the verb 'kunywa' you'll have:
i (I am not drinking)
a (I did not drink)
a (I will not drink)