No quiero meterme en el agua porque hace frío.

Because meterse is a reflexive/pronominal verb here.

• meter = to put, to insert
• meterse = to get into, to go into, to put oneself into

In this sentence, meterme literally means to put myself in / to get into.

The -me means myself:

• meterme = to get myself in
• meterte = to get yourself in
• meterse = to get himself/herself/yourself in

So No quiero meterme en el agua means I don’t want to get into the water.

In Spanish, reflexive pronouns can be attached to an infinitive.

Here:

• quiero meterme = I want to get in

You could also say:

• no me quiero meter en el agua

Both are correct. The meaning is basically the same.

This happens often with infinitives:

• quiero ducharme
• voy a sentarme
• prefiero quedarme

So attaching me to meter is completely normal.

It means to get into the water.

In Spain, meterse en el agua is a very common, natural way to talk about entering water at the beach, pool, sea, river, etc.

Examples:

• No me meto en el agua. = I’m not getting in the water.
• ¿Te vas a meter en el agua? = Are you going to get in the water?

It sounds a bit like go in the water / get in the water in English.

Sometimes, yes, but meterse en el agua is usually more natural here.

• entrar en el agua = to enter the water
• meterse en el agua = to get into the water

Entrar is understandable, but it can sound a bit less idiomatic in this everyday situation. For water, especially at the beach or pool, Spaniards often prefer meterse.

So:

• No quiero meterme en el agua sounds very natural.
• No quiero entrar en el agua is possible, but less common in casual speech.

Because meterse en is the usual structure.

• meterse en algo = to get into something

So:

• meterse en el agua
• meterse en la piscina
• meterse en la cama

You may sometimes hear expressions with al agua, but in this sentence, after meterse, en el agua is the normal choice.

Because hace frío is the standard way to talk about weather or ambient temperature.

• hace frío = it’s cold / the weather is cold
• hace calor = it’s hot
• hace viento = it’s windy

If you say está frío, you are usually talking about a thing being cold:

• El agua está fría. = The water is cold.
• La comida está fría. = The food is cold.

So:

• porque hace frío = because it’s cold
• porque el agua está fría = because the water is cold

Both are correct, but they mean slightly different things.

Because Spanish often leaves out subject pronouns when the verb already makes the subject clear.

• quiero already means I want
• so yo is not necessary

Spanish does this all the time:

• Tengo hambre. = I’m hungry.
• Voy mañana. = I’m going tomorrow.
• No quiero meterme en el agua. = I don’t want to get into the water.

You can add yo for emphasis or contrast:

• Yo no quiero meterme en el agua, pero ellos sí. = I don’t want to get in the water, but they do.

In Spanish, no normally goes directly before the conjugated verb.

So:

• No quiero = I do not want
• No tengo = I do not have
• No puedo = I cannot

In this sentence, the main conjugated verb is quiero, so no goes before it:

• No quiero meterme en el agua

Not:

• Quiero no meterme... unless you mean something more specific like I want not to get in, which is much less natural here.

They are different words.

• porque = because
• por qué = why

In your sentence:

• No quiero meterme en el agua porque hace frío. = I don’t want to get into the water because it’s cold.

A question would use por qué:

• ¿Por qué no quieres meterte en el agua? = Why don’t you want to get into the water?

This is a very common spelling point for learners.

Because in hace frío, frío is not agreeing with a noun like agua. It is part of a fixed weather expression.

So you say:

• hace frío
• hace calor

But if you describe water, then the adjective must agree with agua:

• El agua está fría.

This can be confusing because agua is feminine, but in the singular it usually takes el:

• el agua fría
• el agua está fría

So:

• hace frío = it’s cold
• el agua está fría = the water is cold

Because feminine nouns that start with a stressed a- sound often use el in the singular for pronunciation reasons.

So:

• el agua
• el águila
• el aula

But they are still feminine:

• el agua fría
• el águila blanca

In the plural, they go back to las:

• las aguas
• las águilas
• las aulas

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

Not naturally. A word-for-word translation can help you see the structure, but it often sounds wrong in English.

A rough literal breakdown is:

• No = not
• quiero = I want
• meterme = to put myself / to get myself in
• en el agua = in the water
• porque = because
• hace frío = it is cold

Natural English is:

• I don’t want to get in the water because it’s cold.

So yes, the Spanish structure is a bit different from English, especially with meterme and hace frío.

Yes, very natural.

In Spain, No quiero meterme en el agua porque hace frío sounds like normal everyday speech, especially at the beach or pool.

A few similar natural sentences are:

• No me apetece meterme en el agua.
• No quiero bañarme, hace frío.
• Yo no me meto, que hace mucho frío.

Your original sentence is clear, idiomatic, and perfectly normal.