This question already has an answer here:
According to Duolingo, the phrase "The water is not deep" should be translated "El agua no es profunda." instead of what I guessed, "El agua no es profundo."
Since "agua" is masculine (el), shouldn't the adjective indicate that by having an "o" ending (rather than the feminine "a" ending?
If Duolingo is correct (they probably are), why this seeming anomaly?