It was probably because the question wanted you to specifically say 'We are drinking the water'. Keep in mind that the 'el' in el agua does not relate to feminine/masculine. 'nosotras bebemos agua' can mean 'we drink water' or 'we drink the water', but 'nosotras bebemos el agua' can only mean 'we drink the water'.
tl;dr 'el' in el agua means 'the'
Hope this helps! :)
"Nosotros" is a pronoun and does not have a gender per se, only nouns have gender. It is used for any group of all males or any mixed group of males and females, even if there are 99 males and one female or 99 females and one male. "Nosotras," the other hand, is only used for a group of all females.
"Bebemos" is not masculine. Verbs never have gender. They are conjugated according to tense (present, past, etc.), number (singular or plural), and person (yo, tú, etc.).
"Agua" is a feminine noun, used as a direct object in this sentence. They do not normally change gender unless they are referring back to the subject. For example: Él bebe agua. Ella bebe vino. BUT Él es doctor. Ella es doctora.
Choosing whether to use 'Nosotras' OR 'Nosotros'; does it relate to the thing in question or the person? For example, 'the milk' is 'la leche'...feminine article. So if we are a bunch of MEN ordering MILK, do we say "Nosotros bebemos leche, por favor" or "Nosotras bebemos leche, por favor"?
See my comment about conjugating -er verbs:
it is conjugated differently. beber is an -er verb. so the endings would change depending on the context it is used in. when saying nosotros, you would use the ending -emos (bebemos). when using yo, you would use -o (bebo). when saying él, ella, or usted, you'd use -e (bebe). when using tú, you use -es (bebes). and when using ellos, ellas, ustedes, you'd use -es (bebes).
The two forms are not interchangeable; it depends who is being referenced. It is all down to verb 'conjugation'.
The infinitive of the verb all these derive from is 'Ser (to be)'. From here the verb is conjugated according to who is being referenced. For example: eres = You are somos = We are
It is advisable to spend some time with verb conjugation, especially the common ones . E.g. Tener (to have); Ser (to be); Comer (to eat); Beber (to drink). It takes a little time to learn - particularly the irregular verb conjugations - but it does seem to be very important.
Hope that helps.
"Ustedes" means "you" (plural) and requires "beben" as the conjugated form of the verb "beber." Ustedes beben leche. (You drink milk.) The link below is one of several sites where you can check the conjugation of verbs. You can also look up the definition of words there.
Spanish conjugates its verbs. This means that the verb must agree with the subject in person (I, you, he, we, etc.) and number (singular or plural). "Bebemos" is the conjugation for the subject "we." "Bebes" is the conjugation for the subject "you" (singular informal). See the first link below for how to conjugate ER verbs. The second link shows the full conjugation of the verb "beber." Don't let it scare you! You will be learning it gradually.
"NosOtros" means "we" when it is the subject of the sentence. It can mean "us" when it is the object of a preposition such as: Él agua es para nosotros. (The water is for us.) But be careful of this when you get to object pronouns because nosotros cannot be used as an object pronoun.
It's pronounced more closely like an English /v/.
In Spanish, the letters 'b' and 'v' are the same. If either one appears at the beginning of a word, it should sound like an English /b/, but if it appears anywhere else in the word, its not really going to be pronounced like either letter in English, but you should be able to get away with pronouncing it like /v/.
For anyone confused about the word "bebemos": It comes from the root infinitive, "beber".
The present-tense conjugations are thus (with singular first, then plural):
First-person: "[yo] bebo" -> "I drink" ; "[nosotros/as] bebemos" -> "we drink"
Second-person informal: "[tú] bebes" -> "you drink" ; "[vosotros/as] bebéis" -> "you all drink" (note the vosotros form is only used in Spain)
Second-person formal and third-person: "[usted/él/ella] bebe" -> "you/he/she drinks" ; "[ustedes/ellos/ellas] beben" -> "you all/they drink"