Plus nous approchons de la mer, plus le vent est fort.

This is a very common French pattern: plus..., plus...

It means the more..., the more...

So:

• Plus nous approchons de la mer = The closer / the more we get to the sea
• plus le vent est fort = the stronger the wind is

French often uses plus this way to show that two things increase together.

Here it definitely means more.

French plus can have different meanings:

• plus = more
• ne... plus = no more / no longer

In this sentence, the structure plus..., plus... clearly shows the positive comparative meaning: the more..., the more...

So there is no negative meaning here.

Because this sentence uses the fixed correlative pattern plus..., plus...

In that pattern, plus often introduces the whole clause:

• Plus je lis, plus j’apprends.
• Plus il fait chaud, plus je suis fatigué.

So plus le vent est fort means the stronger the wind is.

If you said le vent est plus fort, that would just be a normal comparison: the wind is stronger. That is grammatical, but it is not the standard way to build the the more..., the more... structure.

Because the verb construction here uses de.

With approcher in this sense, French says:

• approcher de quelque chose = to get close to something

So:

• approcher de la mer
• approcher du village
• approcher de la fin

English learners often expect à, because English says approach the sea or get close to the sea, but in French this verb pattern takes de here.

Approcher de is correct here.

You may also see s’approcher de, which is also very common:

• Nous approchons de la mer
• Nous nous approchons de la mer

Both can mean we are getting closer to the sea.

A useful way to think about it is:

• approcher de = acceptable and common, especially in narrative or descriptive French
• s’approcher de = also very common, often felt as a bit more explicit: to move closer to

So the sentence you have is perfectly grammatical.

Approchons is the present tense, 1st person plural of approcher:

• j’approche
• tu approches
• il/elle approche
• nous approchons
• vous approchez
• ils/elles approchent

The sentence uses the present because it expresses a general relationship or an ongoing situation:

As we get closer to the sea, the wind gets stronger.

French often uses the present for this kind of general truth or immediate observation.

Because vent is a masculine singular noun:

• le vent

After être, the adjective agrees with the noun it describes. So:

• le vent est fort
• la pluie est forte

Here the adjective describes le vent, so it must be fort.

Because fort is an adjective, and here it describes the noun le vent.

• Le vent est fort = The wind is strong

Fortement is an adverb, so it would describe a verb, not the noun:

• Le vent souffle fortement = The wind is blowing strongly

So in your sentence, fort is the correct form.

French normally uses an article with nouns in cases like this.

So French says:

• la mer = the sea
• la montagne = the mountain
• la ville = the town/city

Even when English might sound more flexible, French usually needs the article.

So de la mer is the normal form here.

Because de only contracts with le and les:

• de + le = du
• de + les = des

But:

• de + la = de la
• de + l’ = de l’

Since mer is feminine:

• la mer
• de la mer

So du mer is impossible.

Here plus is pronounced with the final s:

• /plys/

That is normal in this comparative/correlative use.

So the sentence begins roughly like:

• Plus nous approchons... → /plys nu za.pʁɔ.ʃɔ̃/

This is different from negative ne... plus, where the final s is often not pronounced in everyday speech.

The comma is very natural here because it separates the two linked parts of the plus..., plus... structure:

• Plus nous approchons de la mer, plus le vent est fort.

In writing, this comma is standard and helps readability.

You may sometimes see short plus..., plus... sentences written without a comma, but using one here is the normal and clearest choice.