Deniz suyu çok soğuk olursa, denize girmeyiz.

It’s the conditional marker “if.” Here olur is the simple-present 3rd-person form of olmak (“to be”), and adding -sa (vowel-harmonized) turns it into “if it is.”

-sa/-se is one suffix. You pick -sa after back vowels (a, ı, o, u) and -se after front vowels (e, i, ö, ü). Since olur ends in u (a back vowel), it takes -sa.

Yes. Turkish often drops parts of the “if” clause when context is clear. Soğuksa… (“if it’s cold…”) or even Deniz soğuksa… works perfectly well.

There are two patterns:
1) A genitive-possessive construction: denizin suyu (“the sea’s water”).
2) A compound noun without any case on the first word: deniz suyu (“sea water”). The compound is more natural for a fixed concept like water in the sea.

The verb girmek (“to enter”) requires its object in the dative to show direction “into.” If you were simply “loving the sea,” you’d say denizi (accusative). But “enter the sea” needs denize.

In Turkish conditionals, the aorist (simple present) often carries future meaning in the result clause. So girmeyiz literally “we do not enter” is understood as “we will not enter.” You can say girmeyeceğiz too, but the aorist is more idiomatic here.

Breakdown:

• gir- (stem “enter”)
• -me- (negative)
• -r- (aorist/simple-present marker)
• -iz (1st-person-plural ending)
  A buffer y appears between me and iz: gir-me-y-iz → girmeyiz.