El agua me llega hasta la cintura.

Because me shows to me / on me / as far as my body is concerned.

In this sentence, llegar is being used in the sense of to reach. So El agua me llega hasta la cintura literally works like:

• The water reaches me up to the waist

That me is an indirect object pronoun, and it is very common when talking about how something affects or reaches part of someone’s body.

Similar examples:

• El agua me llega a las rodillas. = The water comes up to my knees.
• La falda me llega hasta los tobillos. = The skirt reaches my ankles.

Without me, the sentence would sound incomplete or would no longer clearly express that the water reaches your waist level.

Yes, llegar often means to arrive, but it also means to reach.

Here it means to reach a certain height or point. That is a very common use.

So:

• El agua llega hasta la cintura = The water reaches up to the waist.
• La mesa me llega al pecho. = The table comes up to my chest.

This is a normal Spanish way to talk about level or height. English often uses come up to or reach in the same way.

Here, hasta means up to / as far as.

So:

• hasta la cintura = up to the waist

It marks the limit or end point.

More examples:

• hasta las rodillas = up to the knees
• hasta el cuello = up to the neck
• hasta aquí = up to here

In this sentence, hasta tells you the highest point the water reaches.

Spanish often uses the definite article with body parts where English prefers a possessive.

So Spanish says:

• me llega hasta la cintura literally it reaches me up to the waist

English usually says:

• it comes up to my waist

Because me already tells us whose waist is involved, Spanish normally uses la cintura, not mi cintura.

This pattern is very common with body parts:

• Me duele la cabeza. = My head hurts.
• Se lavó las manos. = He washed his hands.
• Le vi la cara. = I saw her face.

Using mi cintura is not impossible, but it is less natural here.

Yes. La cintura means waist.

So:

• hasta la cintura = up to the waist / waist-deep

Related body-part words you might also see:

• el tobillo = ankle
• la rodilla = knee
• la cadera = hip
• el pecho = chest
• el cuello = neck

A very common pattern is:

• El agua me llega + a/hasta + body part

Yes, you can. Both are natural, though there can be a slight nuance.

• me llega a la cintura = it reaches my waist
• me llega hasta la cintura = it comes up as far as my waist / up to my waist

In many everyday situations, they are very close in meaning.
Hasta can sound a little more explicit about the upper limit.

You will hear both:

• El agua me llega a las rodillas.
• El agua me llega hasta las rodillas.

Good question: agua is actually a feminine noun, but it takes el in the singular when a stressed a- sound comes first.

So we say:

• el agua fría not la agua fría

Notice the adjective is still feminine:

• fría, not frío

This happens with some other feminine nouns too:

• el alma
• el águila

But in the plural, they go back to las:

• las aguas
• las águilas

So in your sentence, el agua is correct even though agua is feminine.

Yes. That is a very natural English translation.

Spanish often expresses this idea with a full verb phrase:

• El agua me llega hasta la cintura.

English may translate it as:

• The water comes up to my waist.
• The water reaches my waist.
• The water is waist-deep.

All of these fit the same general meaning.

Yes. Spanish word order is flexible.

The neutral order is:

• El agua me llega hasta la cintura.

But you might also hear:

• Me llega el agua hasta la cintura.
• Hasta la cintura me llega el agua.
  This sounds more marked or emphatic.

The standard version is the best one for learners to use first.

You change the pronoun:

• El agua me llega hasta la cintura. = The water comes up to my waist.
• El agua te llega hasta la cintura. = The water comes up to your waist.
• El agua le llega hasta la cintura. = The water comes up to his/her/your (formal) waist.
• El agua nos llega hasta la cintura. = The water comes up to our waist.
• El agua os llega hasta la cintura. = The water comes up to your waist. (Spain plural)
• El agua les llega hasta la cintura. = The water comes up to their/your (formal plural) waist.

This is the same body-part pattern: pronoun + article + body part.

Yes, absolutely. It sounds natural and idiomatic.

A speaker from Spain might use this in situations like:

• at the beach
• crossing a river
• describing flooding
• talking about swimming depth

Other very natural variants in Spain include:

• El agua me llega por la cintura.
• Tengo el agua por la cintura.
• El agua me cubre hasta la cintura.

But El agua me llega hasta la cintura is completely standard and natural.