Кит плавает в море.

This is a very common question.

• плавает usually means swims / is swimming around / swims habitually
• плывёт usually means is swimming right now in one direction

So:

• Кит плавает в море. = a general fact, or a neutral description
• Кит плывёт к берегу. = the whale is swimming toward the shore right now

In a simple example sentence like this, плавает sounds natural because it describes what a whale does in general.

Because в means in here, and when Russian shows location after в, it normally uses the prepositional case.

So:

• в море = in the sea

A useful contrast:

• в море = in the sea
• в море? location
• в море can look the same as the dictionary form here, because море is one of those nouns whose nominative and prepositional singular look identical

So the case has changed grammatically, even though the form happens to stay море.

It actually is in the correct case already: prepositional singular.

The noun море is neuter, and its forms are:

• nominative: море
• prepositional: о море, в море

So in this noun, the nominative and prepositional singular are the same in spelling. That is why you do not see a visible change.

It can mean either.

Russian has no articles like a or the. So:

• Кит плавает в море. could mean A whale swims in the sea
• or The whale is swimming in the sea

Context tells you which one is meant.

In a basic textbook sentence, it is often understood as a general statement: A whale swims in the sea or The whale is in the sea / swims in the sea, depending on context.

A rough English-friendly guide is:

• Кит = keet
• плавает = PLA-va-yet
• в море = v MO-rye

Stress:

• кит
• пла́вает
• мо́ре

A more careful pronunciation is approximately:

keet PLA-va-yit v MO-rye

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

The neutral order here is:

• Кит плавает в море.

But you could also hear:

• В море плавает кит.
• Плавает кит в море.

These versions change the focus or emphasis, not the basic meaning.

For example:

• В море плавает кит. puts more attention on where
• Кит плавает в море. is the most straightforward, neutral version

Yes, кит is masculine.

A good basic rule is:

• nouns ending in a hard or soft consonant are often masculine
• nouns ending in -а / -я are often feminine
• nouns ending in -о / -е are often neuter

So:

• кит → masculine
• море → neuter

In this sentence, the present-tense verb form плавает does not clearly show masculine gender, but in the past tense it would:

• Кит плавал. = The whale swam.
  Here плавал shows masculine.

It is imperfective.

That matters because imperfective verbs are used for things like:

• ongoing action
• repeated action
• habitual action
• general facts

This sentence is a general statement, so imperfective is exactly what you would expect.

The perfective partner of плавать is not normally used for a general truth like this.

Both в and на can sometimes relate to the sea, but they mean different things.

• в море = in the sea
• на море = often at the seaside / by the sea / on holiday at the sea

So in this sentence, the whale is physically in the sea, so в море is correct.

Examples:

• Кит плавает в море. = The whale swims in the sea.
• Мы отдыхаем на море. = We are on holiday at the seaside.

By itself, it could be understood either way, depending on context.

It may mean:

• a specific whale: The whale is swimming in the sea
• a general example: A whale swims in the sea

If you want a clearly general plural statement, Russian often uses the plural:

• Киты плавают в море. = Whales swim in the sea.

So the singular can be a simple example sentence, while the plural sounds more clearly like a general fact about whales as a class.