Ako budemo išli na plažu, ponijet ću vodu i ručnik.

Because Croatian normally uses future II in an ako clause when you are talking about a future condition.

So the pattern is:

• Ako budemo išli... = If we go / If we end up going...
• ponijet ću... = I will bring...

Using ćemo ići after ako sounds unnatural here.
A useful thing to remember is:

• in English: If we go...
• in Croatian: often Ako budemo išli...

Also, ići is an imperfective verb, so Croatian uses future II here. With a perfective verb, Croatian often uses the present form instead, for example Ako odemo...

It is made from:

• budemo = the auxiliary from biti used for future II
• išli = the l-participle of ići

So:

• budem išao/išla
• budeš išao/išla
• bude išao/išla
• budemo išli/išle
• budete išli/išle
• budu išli/išle

In your sentence, budemo išli means we will be going / if we go in this conditional-future sense.

A small extra detail: išli is the masculine/mixed plural form. If the group were entirely female, you could hear budemo išle.

This is the future I form of ponijeti.

Croatian future I is built with:

• the infinitive
• plus a short form of htjeti: ću, ćeš, će, ćemo, ćete, će

So:

• ponijeti
  • ću → ponijet ću

When the auxiliary comes after the infinitive, the final -i of the infinitive drops:

• ponijeti → ponijet ću
• raditi → radit ću

But if the auxiliary comes first, the full infinitive stays:

• Ja ću ponijeti vodu i ručnik.

Both are normal.

Because Croatian often leaves out subject pronouns when the verb already shows who the subject is.

Here:

• budemo already tells you we
• ću already tells you I

So Croatian does not need mi or ja unless you want emphasis or contrast.

For example:

• Ako budemo išli na plažu, ponijet ću vodu i ručnik. = neutral
• Ako mi budemo išli na plažu, ja ću ponijeti vodu i ručnik. = more emphatic, like if we are the ones going... I will bring...

Because na can take different cases depending on whether you mean:

• movement toward a place → accusative
• location at a place → locative

Here the idea is going to the beach, so Croatian uses na + accusative:

• na plažu = to the beach

Compare:

• Idemo na plažu. = We’re going to the beach.
• Mi smo na plaži. = We are at the beach.

So:

• plaža = nominative
• plažu = accusative
• plaži = locative

Both words are direct objects, so they are in the accusative case.

• voda → vodu
• ručnik → ručnik

The reason they look different is that Croatian noun patterns depend on gender and animacy.

voda is feminine, so its accusative singular changes to vodu.

ručnik is masculine inanimate, and masculine inanimate nouns usually have the same form in nominative and accusative singular, so it stays ručnik.

Compare:

• Imam vodu.
• Imam ručnik.

But with a masculine animate noun, the accusative usually changes:

• vidim čovjeka = I see the man

Ponijeti means something like to take/bring along with you.

In this sentence, the speaker is saying they will bring those items along when going to the beach. That is why ponijeti fits very naturally.

It is slightly different from some related verbs:

• ponijeti = take/bring along
• donijeti = bring to a destination / bring here-there
• nositi = carry, wear, or carry habitually/ongoingly

So:

• Ponijet ću vodu i ručnik. = I’ll bring/take along water and a towel.
• Donijet ću ti vodu. = I’ll bring you water.
• Nosim ručnik. = I’m carrying a towel / I carry a towel.

Yes, it matters.

Ponijeti is a perfective verb, which means it presents the action as a completed whole: bringing the water and towel as one complete act.

That is very natural in a future sentence like this, because the speaker means a single completed action in the future.

If you used the imperfective nositi, it would sound more like repeated, habitual, or ongoing carrying:

• ponijet ću = I will bring / I’ll take along
• nosit ću = I will be carrying / I’ll carry habitually

So ponijet ću is the better choice here.

Yes. Croatian word order is more flexible than English word order.

The sentence you have is a very natural, neutral version:

• Ako budemo išli na plažu, ponijet ću vodu i ručnik.

You could also say:

• Ponijet ću vodu i ručnik ako budemo išli na plažu.
• Vodu i ručnik ponijet ću ako budemo išli na plažu.

The meaning stays very similar, but the focus can shift a little.

One important detail: ću is a clitic, and clitics usually appear near the beginning of their clause, often in second position. That is why you get patterns like:

• ponijet ću
• ja ću ponijeti

Yes, in this sentence the comma is standard and expected.

The first part:

• Ako budemo išli na plažu

is a subordinate clause, and it comes before the main clause:

• ponijet ću vodu i ručnik

So Croatian normally separates them with a comma:

• Ako budemo išli na plažu, ponijet ću vodu i ručnik.

If you reverse the order, the punctuation may change:

• Ponijet ću vodu i ručnik ako budemo išli na plažu.

A rough guide:

• č = like ch in church, but a bit firmer
• ć = a softer sound; many English speakers hear it as something between ty and a soft ch
• ž = like the s in measure

In this sentence:

• ručnik ≈ ROOCH-neek
• ću ≈ a soft tyoo/chyoo
• plažu ≈ PLAH-zhoo

English does not have perfect matches for all of these sounds, so approximation is normal at first.

A very common learner issue is mixing up č and ć. Native speakers do hear the difference, but even if your pronunciation is not perfect yet, you will usually still be understood.