Learn Hardcore Hungarian: Ma csak vizet iszom.

stem change + linking vowel + suffix: víz → viz + -et = vizet Two things happen:
The long vowel í shortens to i in this stem.
A linking vowel -e- appears before -t (chosen by vowel harmony because víz is a front-vowel word).

Examples:

Without article (generic): vizet iszom
With article (specific): a vizet iszom

Is spelling it vízet ever correct?

No. The correct accusative is vizet (short i). You will see both stems with this noun:

Shortened stem: vizet, vizek, vizem
Original stem in some forms: vízzel (with -val/vel), vízben (inessive) So the alternation is normal, but the accusative is always vizet.

Why is there no article before vizet?

No article signals an indefinite, generic object: “water (in general).” If you say a vizet, you mean a specific water (for example, the water we poured earlier):

Ma csak vizet iszom. = Today I drink only water (as a type).
Ma csak a vizet iszom. = Today I drink only the water (the specific one), not the juice, tea, etc.

What does csak do, and where should it go?

Csak means “only” and directly precedes the element it limits. Hungarian focus (the most emphasized, contrastive part) appears immediately before the verb. In this sentence, the focused phrase is csak vizet and it stands right before iszom:

Neutral, natural: Ma csak vizet iszom.
Also fine (slightly different rhythm): Csak vizet iszom ma. Don’t split csak from what it limits:
Ungrammatical/odd: ✗ Ma vizet csak iszom. (csak is separated from its focus)

Could I say Ma csak iszom vizet?

That changes the focus. With csak directly before the verb, the notion becomes “I only drink,” and vizet is just what you’re drinking, not the limited set. To say “only water,” keep csak with vizet and put them immediately before the verb:

Best: Ma csak vizet iszom.

Why is it iszom and not iszok?

Standard Hungarian uses iszom for “I drink.” Many speakers say iszok colloquially, but it’s nonstandard/less formal. For learners (and in writing), stick to iszom.

What exact verb form is iszom?

It’s 1st person singular, present tense of iszik (“to drink”). Technically it’s the indefinite conjugation, but in 1st person singular this verb’s definite and indefinite forms are identical: iszom either way. You see the difference in other persons:

Indefinite 2sg: iszol vizet (“you drink water”)
Definite 2sg: iszod a vizet (“you drink the water”)

Is this “I drink” or “I am drinking” in English?

Hungarian present covers both simple and progressive aspects. Ma (“today”) supplies the time frame, so Ma csak vizet iszom can mean either “I drink only water today” or “I’m only drinking water today,” depending on context.

How would I negate this while keeping the “only water” idea?

Put nem before the verb and use mást (“something else/other”) to keep the “only” restriction:

Ma nem iszom mást, csak vizet. = Today I don’t drink anything else, only water. If you say Ma csak vizet nem iszom, it means “It’s only water that I don’t drink today” (implies you may drink other things), which is a different meaning.

Can I change the position of ma?

Yes. Ma is a time adverb and is flexible:

Ma csak vizet iszom. (neutral)
Csak vizet iszom ma. (also fine) If you move csak to modify ma instead, you change the meaning:
Csak ma vizet iszom. = Only today do I drink water (implies that on other days I don’t).

What’s the difference between csak and egyedül?

csak = “only” (restricts category/quantity: only water).
egyedül = “alone/by myself.” So Ma csak vizet iszom is about what you drink, while Ma egyedül iszom says you’re drinking alone.

Any quick pronunciation tips for this sentence?

Stress is always on the first syllable of each word: MA csak VI-zet I-szom.
cs = “ch” in “chocolate.”
sz = “s” in “see” (note: Hungarian s alone is “sh”).
Vowels: a is like a back “aw/ah” [ɒ]; o is a round “o” [o]; i is [i]. So roughly: [mɒ t͡ʃɒk ˈvizɛt ˈisom].

Why does víz sometimes keep the long í and sometimes shorten to i?

This noun has two stems. With some suffixes you get the short-stem viz- (e.g., vizet, vizek, vizem), and with others you keep víz- (e.g., vízben, vízzel). It’s lexical and common in Hungarian; just learn the main forms as you encounter them.

Can I use a perfective prefix like meg- here?

Not with a bare, indefinite object. Meg- usually needs a definite object when you mean “drink up/finish”:

Megiszom a vizet. = I’ll drink up the water (specific). Your sentence, Ma csak vizet iszom, talks about what you (will) drink today in general, not completing a specific quantity.

How would I say this as a general habit?

Simple: Csak vizet iszom. = I only drink water.
To emphasize habit, Hungarian often uses szoktam with the infinitive: Csak vizet szoktam inni. = I usually/typically drink only water.