Deniz suyu tuzlu.

In Turkish, the present-tense copula ("to be") is usually omitted in simple nominal sentences.
So Deniz suyu tuzlu literally reads "sea water salty" but is understood as "sea water is salty."
If you want to be more formal or explicit, you can add the suffix -dur: Deniz suyu tuzludur ("Sea water is salty").

The suffix -lu (with vowel harmony: -lı/-li/-lu/-lü) attaches to nouns to form adjectives meaning "with" or "having."
For example:
• tuz (salt) + -lu → tuzlu ("salty," literally "having salt")
• şeker (sugar) + -li → şekerli ("sweet," "having sugar")

In standard noun-noun compounds, you don’t add the genitive suffix to the first noun. Instead the relationship is shown by adding the possessive suffix to the second noun.
However, you can express the same meaning more explicitly with a true genitive + possessive construction:
Deniz+in (genitive) → denizin, then su+yu (possessive) → suyu → Denizin suyu.
Both mean "the sea’s water," but deniz suyu is the common compound form.

Turkish does not have separate words for definite (the) or indefinite (a/an) articles.
Definiteness is often inferred from context or indicated by case endings.
Here Deniz suyu tuzlu makes a general statement ("Sea water is salty") without needing an article.

Turkish generally follows Subject-Object-Verb (SOV) order, but in a sentence with a predicate adjective, you have:
Subject + (Object) + Predicate.
Since tuzlu is the predicate adjective, it comes at the end:
Deniz suyu (subject) + tuzlu (predicate).

Add the question particle -mu/-mü/-ma/-me (with vowel harmony) after the predicate adjective:
Deniz suyu tuzlu mu?
Literally: "Sea water salty, yes/no?"

Use değil after the predicate adjective to make a negative nominal sentence:
Deniz suyu tuzlu değil.
Literally: "Sea water salty not," meaning "Sea water is not salty."