nước is a classifier for liquids, as far as i understand. It is also used as a classifier for nations as well.
Yeah, nước is a classifier for beverages and it is also the word for water. It depends on the context of the sentence as well as what words are around nước. In this case, we see that nước is followed by trà. So we know we're really talking about tea. Nicolas is also right to say that it's a classifier for nations too.
Why is rắn pronounced with a z sound? Is this a northern dialect thing? Would it be the same in southern dialect?
Amisha is right to say that the "r" makes a "z" sound in the northern dialect. However, in the south, the "r" makes a true "r" sound like in English. It's Vietnamese words that start with "gi" and "d" that make a "y" sound (as in "you") in the southern dialect. Hope that helps =)
When pronouncing the "r" with a "z" sound, it is a northern dialect. I think southern dialect would pronounce the "r" with a "y" or "g" sound.