"El agua está profunda."
That makes good sense. I know that estar is used in place of ser for properties that are unusual. For example: La nieve es blanca. (Snow is white.) La nieve está amarilla. (The snow is yellow.)
So I too guess the water is not always/usually deep.
The moral of the story is ...Don't eat yellow snow. ;)
I just noticed something I haven't noticed on DL before: detailed explanations of grammar. In this case, I hovered over the word "El" in "El agua esta profunda" and got this: "El is often used for masculine nouns and la for feminine ones. But when the noun begins with a stressed a- or ha-, you must use el regardless of the gender. For example, agua is a feminine noun, but you say el agua and not la agua." Bravo, DL!