Eu vou colocar gelo na água agora.

Yes. In Brazilian Portuguese, eu can often be dropped because the verb form already shows the subject:

• (Eu) vou colocar gelo na água agora.

Both are correct.
Including eu can make the sentence a little clearer, more emphatic, or more natural in some contexts, especially in speech.

Vou colocar is the very common ir + infinitive future structure:

• vou = I go / I am going
• colocar = to put

Together, vou colocar means I’m going to put.

This is one of the most common ways to talk about the near future in Brazilian Portuguese, just like in English:

• Eu vou colocar gelo... = I’m going to put ice...

A more formal or less common one-word future would be:

• colocarei = I will put

But in everyday Brazilian Portuguese, vou colocar is much more natural.

Yes. All three can mean to put:

• colocar
• pôr
• botar

In Brazil:

• colocar is very common and neutral
• botar is also very common in speech and sounds a bit more informal
• pôr is correct but can feel a little less common in some everyday contexts, depending on the region

So these are all possible:

• Eu vou colocar gelo na água agora.
• Eu vou botar gelo na água agora.
• Eu vou pôr gelo na água agora.

For learners, colocar is a very safe and useful choice.

Because gelo is being used as a mass noun here, like ice in English.

• colocar gelo = to put ice
• not necessarily the ice or some specific ice

If you were talking about specific ice already mentioned, you could use an article:

• Vou colocar o gelo na água. = I’m going to put the ice in the water.

But in this sentence, gelo is more general, so no article is very natural.

Because Portuguese normally contracts em + a into na.

So:

• em + a = na
• em + o = no
• em + as = nas
• em + os = nos

That means:

• na água = in the water

So Eu vou colocar gelo na água agora literally contains:

• colocar = put
• gelo = ice
• na água = in the water

This kind of contraction is very common and important in Portuguese.

Yes, água is feminine.

You say:

• a água
• na água
• água gelada = cold water
  not água gelado

Even though água begins with a stressed a, it is still feminine. Portuguese keeps the feminine article:

• a água
• uma água

This is normal. The important thing is that adjectives agree in the feminine:

• água fria
• água limpa

So in your sentence, na água is feminine because it comes from em + a água.

This is a very common learner question.

In this sentence:

• gelo is being treated as a general substance, so no article is needed
• água appears with the article because Portuguese often uses articles in places where English does not, especially with nouns after prepositions

So:

• gelo = ice
• na água = in the water

Portuguese and English do not always match article usage exactly.
A sentence that sounds natural in Portuguese may use an article where English would not.

Agora means now.

It tells you the action is happening now / right now / at this moment:

• Eu vou colocar gelo na água agora. = I’m going to put ice in the water now.

It gives the sentence a sense of immediacy.

Without agora, the sentence would still be correct:

• Eu vou colocar gelo na água. = I’m going to put ice in the water.

Adding agora makes it more immediate.

Yes, agora is flexible.

These are all possible:

• Eu vou colocar gelo na água agora.
• Agora eu vou colocar gelo na água.
• Eu agora vou colocar gelo na água. (less common, more marked)

Putting agora at the end is very natural and common.
Putting it at the beginning often adds a bit more focus to now.

Here, na água is the correct choice.

• na água = in the water
• à água is a different structure and would not mean the same thing here

Since you are literally putting ice into the water, the preposition em is the normal one:

• colocar gelo na água

So for this idea, stick with na água.

Yes. Even though the literal structure is put ice in the water, the natural English idea is often:

• I’m going to add ice to the water now
• I’m going to put ice in the water now

Both match the Portuguese sentence well.

Portuguese often uses very direct verbs like colocar where English might choose either put or add, depending on the context.

Água is usually pronounced roughly like:

• AH-gwa

A few details:

• the stress is on the first syllable: Água
• the gu in água sounds like gw
• the final a is a light vowel sound

So:

• a água
• na água

When spoken naturally, na água may sound smoothly connected, almost like one unit.

Yes, it sounds natural.

A Brazilian might also say:

• Vou colocar gelo na água agora.
• Vou botar gelo na água agora.
• Agora vou colocar gelo na água.

All of these are normal. Your original sentence is clear, correct, and natural.

You can understand it word by word, but a fully natural English translation may vary.

Word by word:

• Eu = I
• vou colocar = am going to put
• gelo = ice
• na água = in the water
• agora = now

So the literal translation is:

• I’m going to put ice in the water now.

But depending on context, natural English might also be:

• I’m going to add ice to the water now.

So the structure is easy to follow, but the most natural English wording can change a little.