I'm getting tired of this definite article stuff. We don't use them in English unless we're being specific. Since the water is not being specified as a particular "water," there is no need to call it "the water" in English.
Without specification of the object, the translation is simply, "we drink water. "
Het and de are both translated to English as 'the'. For the singular versions of the words there is no logic which is which (apparently 'de' is for masculine and feminine words, and 'het' is for neuter words), you'll just have to learn it. For certain forms of the nouns, there are rules though. All plural words have de, and all small words (ending in -je, see one of the next lessons) have het.