Zimi ne plivam u moru, ali bih plivala da je voda toplija.

Yes. Zimi means in winter / during the winter.

It comes from zima (winter), but here it is used in a fixed adverbial way, so Croatian does not need a preposition like English in.

So:

• Zimi ne plivam = I don’t swim in winter

This is a very natural Croatian time expression.

Because Croatian uses the present tense for habits and general truths, just like English often does.

So ne plivam here does not mean only I am not swimming right now. It means:

• I don’t swim
• I’m not someone who swims
• I don’t usually swim

With zimi, the meaning becomes a habitual one:

• Zimi ne plivam = In winter, I don’t swim

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

The ending in plivam tells you it is I:

• plivam = I swim

So ja is unnecessary unless you want emphasis or contrast.

Compare:

• Ne plivam. = I don’t swim.
• Ja ne plivam. = I don’t swim (with extra emphasis on I)

Because u changes case depending on meaning.

Here it means location: in the sea, so Croatian uses the locative:

• u moru = in the sea

The noun more becomes moru in the locative singular.

Very roughly:

• u moru = in the sea (location)
• u more = into the sea (movement toward the inside)

Since the sentence is about swimming in a place, u moru is the correct form.

Bih plivala means I would swim.

It is the Croatian conditional. It is formed with:

• the auxiliary bih
• plus the l-participle of the main verb

So:

• bih plivala = I would swim

Other examples:

• bih išla = I would go (female speaker)
• bih rekla = I would say (female speaker)

This is one of the most common ways Croatian expresses would.

Because the participle agrees with the speaker’s gender.

• plivala = said by a female speaker
• plivao = said by a male speaker

So this sentence tells you the speaker is female:

• ali bih plivala = but I would swim (female speaker)

A man would say:

• Zimi ne plivam u moru, ali bih plivao da je voda toplija.

Because bih is a clitic, and clitics in Croatian usually go in the second position of the clause.

In this clause:

• ali bih plivala

the word ali comes first, so the clitic bih naturally comes right after it.

This word order is very normal and neutral.

You may also hear other orders for emphasis, but ali bih plivala is the standard, straightforward version.

Good question. Da often does mean that, but in some sentences it can also introduce a condition, especially a hypothetical one.

Here:

• da je voda toplija = if the water were warmer

So in Croatian, da can function like if, depending on context.

This is especially common in sentences with a conditional meaning in the main clause, such as:

• bih plivala = I would swim

Croatian does not copy the English were structure exactly.

In this sentence, the hypothetical meaning is understood from the whole pattern:

• bih plivala = conditional in the main clause
• da je voda toplija = the imagined condition

So even though je is formally the present tense of biti (is), the sentence as a whole means a hypothetical situation.

A very common alternative is:

• kad bi voda bila toplija

That also means:

• if the water were warmer

So Croatian can express this kind of unreal present condition in more than one natural way.

Toplija is the comparative form of topla (warm), so it means warmer.

It also agrees with voda, which is feminine singular:

• voda = feminine singular
• toplija = feminine singular comparative

So:

• voda je toplija = the water is warmer

In the sentence, it means:

• da je voda toplija = if the water were warmer

Usually, ako is more typical for a real or possible condition, while da or kad bi often sound better for a more hypothetical or unreal one.

So in this sentence:

• da je voda toplija
• kad bi voda bila toplija

fit the meaning if the water were warmer very well.

Using ako is not the most natural choice for this particular hypothetical idea. A Croatian speaker would normally prefer the structure used in your sentence, or kad bi....