Is water countable or uncountable?

[Phoo]

I found an old saying in my dictionary as below.

"Still waters run deep."

Why is the "water" plural?
I have learnt that "water" is uncountable noun.

Can I also say;
"Still water runs deep." or
"A still water runs deep." instead?

What the difference b/w them then?

[Jamie (K)]

Water as a substance that you drink is uncountable. In certain limited usages, we poetically talk about the water in rivers, streams, lakes and oceans in the plural, as "waters". We say "the waters of the Atlantic", "the waters of the Mississippi", etc.

[Phoo]

OK, "poetically" is the key word! I've got it, thanks!