Eu preciso de muita água.

Yes. In Brazilian Portuguese, subject pronouns are often omitted when the verb form already makes the subject clear.

So both of these are natural:

• Eu preciso de muita água.
• Preciso de muita água.

Including eu can add emphasis, contrast, or clarity, but it is not always necessary.

Preciso is the first-person singular form of the verb precisar, which means to need.

Conjugation in the present tense:

• eu preciso = I need
• você precisa = you need
• ele/ela precisa = he/she needs
• nós precisamos = we need
• vocês precisam = you all need
• eles/elas precisam = they need

So Eu preciso means I need.

Because the verb precisar is commonly used as precisar de when it means to need something.

So:

• preciso de água
• precisamos de ajuda
• ela precisa de descanso

For learners, the safest rule is: learn it as precisar de.

In everyday speech, some Brazilians may sometimes omit de, especially in informal language, but standard Portuguese strongly prefers de here.

Because muita agrees with água, and água is a feminine noun.

• muito = masculine singular
• muita = feminine singular
• muitos = masculine plural
• muitas = feminine plural

Examples:

• muito dinheiro
• muita água
• muitos livros
• muitas pessoas

Since água is feminine singular, the correct form is muita.

Yes, água is feminine.

You can see that in phrases like:

• a água
• água gelada
• muita água

A detail that often confuses learners: in the singular, Portuguese uses a before água even though both words start with an a sound. That is normal:

• a água
• a alma
• a área

But in your sentence, there is no article. It is not a água here, just água, because the phrase is talking about an uncountable amount of water in a general way.

Because água is being used as an uncountable noun in a general sense.

Compare:

• Eu preciso de água. = I need water.
• Eu preciso da água. = I need the water.

In de muita água, you are talking about a lot of water, not a specific water already identified in the conversation. So no article is used.

In standard Portuguese, no. The expected form is:

• Eu preciso de muita água.

That is because precisar normally takes de when it means to need.

You may hear omission of de in some informal speech, but as a learner, you should use precisar de.

Yes, it is natural and correct.

A very common alternative in spoken Brazilian Portuguese is:

• Estou precisando de muita água.

This can sound a bit more immediate or situational, like I’m needing / I really need a lot of water right now.

In general:

• Eu preciso de muita água. = simple, direct, correct
• Estou precisando de muita água. = very common in conversation, often more immediate

A simple approximate pronunciation is:

eh-ooh preh-SEE-zoo djee MWEEN-tah AH-gwah

A few notes:

• eu sounds roughly like eh-ooh in Brazilian Portuguese
• preciso has stress on ci: pre-CI-so
• de often sounds like djee before certain sounds in many Brazilian accents, though pronunciation varies
• muita often sounds close to MWEEN-ta
• água has stress on the first syllable: Á-gua

Pronunciation changes somewhat by region, so exact sounds may vary.

Yes, but the normal order is:

• Eu preciso de muita água.

You can also say:

• Preciso de muita água.

Other word orders are possible for emphasis, but they are less neutral:

• De muita água eu preciso. = more literary or emphatic

For everyday speech, stick with Eu preciso de muita água or Preciso de muita água.

Very easily.

Question:

• Eu preciso de muita água? = Do I need a lot of water?
• Preciso de muita água? = Do I need a lot of water?

Negative:

• Eu não preciso de muita água. = I don’t need a lot of water.

In Portuguese, questions are often formed just by intonation, and negation is usually made with não before the verb.

You would change the quantity word:

• Eu preciso de mais água. = I need more water.
• Eu preciso de água. = I need water.
• Eu preciso de um pouco de água. = I need a little/some water.
• Eu preciso de muita água. = I need a lot of water.

This is a useful pattern:

• precisar de + noun
• precisar de + quantity + noun

So once you know Eu preciso de muita água, you can build many similar sentences.