Why is "el agua poco profunda" and not "profundo"? Agua is masculine although it ends with an "a". And from all what i've learnt so far it should be profundo.
Agua is feminine.
The key here is singular feminine nouns that starts with stressed 'a' or 'ha'. Examples: el alma viva, el arma peligrosa, el hambre canina, el habla clara, el hacha pesada, el águila altiva, el ala blanca.
Then the noun is feminine, and the adjective must be feminine although the article is masculine. And only when the article is immediately followed by the noun.
Better and deeper explanation (Spanish).
Agua has the thonic sylabe in the A, words starting like that became "masculine" but just in singular form:
El Agua, El águila, El aula, El arma.
Las aguas, las águilas, las aulas, las armas.
That's because it would sound "lagua". That doesn't sound good. But sometime you will use "la" with those words but it's better if you use "el".