Quand vous serez en vacances, vous pourrez vous baigner dans la mer chaque matin.

French often uses the future tense in both parts of a sentence when both actions are in the future.

So French says:

• Quand vous serez en vacances... = When you are on vacation...
  but literally closer to When you will be on vacation...
• ...vous pourrez... = ...you will be able to...

This is a very common difference between French and English:

• English: When you arrive, I will help you.
• French: Quand tu arriveras, je t’aiderai.

So after quand, French can use the future if the meaning is future.

Serez is the future tense of être (to be) for vous.

The verb is:

• être = to be
• vous serez = you will be

Examples:

• Vous serez prêts. = You will be ready.
• Vous serez en vacances. = You will be on vacation.

It is an irregular verb, so you just have to learn the future form.

Pourrez is the future form of pouvoir (to be able to / can).

• vous pouvez = you can
• vous pourrez = you will be able to

In English, can does not really have a normal future form, so we usually say will be able to. French uses the future of pouvoir instead.

So:

• vous pourrez vous baigner = you will be able to swim / bathe

En vacances is a fixed expression meaning on vacation or on holiday.

French often uses en in set expressions describing a state or condition:

• en vacances = on vacation
• en retard = late
• en forme = in good shape

So you should learn être en vacances as one chunk:

• Je suis en vacances. = I’m on vacation.
• Quand vous serez en vacances... = When you’re on vacation...

Because se baigner is the normal verb for to bathe / to go swimming / to swim for pleasure.

There is an important difference:

• baigner without se usually means to bathe something/someone or appears in more literary uses
• se baigner means to bathe oneself / to swim

So in this sentence:

• vous pourrez vous baigner = you will be able to swim / bathe

The reflexive pronoun vous is part of the verb se baigner.

Because with verbs like pouvoir, vouloir, aller, etc., the reflexive pronoun usually goes before the infinitive.

So:

• vous pourrez vous baigner
• not vous pourrez baigner vous

Compare:

• Je vais me reposer. = I’m going to rest.
• Nous pouvons nous asseoir. = We can sit down.
• Vous pourrez vous baigner. = You will be able to swim.

This is a very common French structure: conjugated verb + reflexive pronoun + infinitive

No. In standard French, each finite verb normally needs its own explicit subject.

So French repeats vous:

• Quand vous serez en vacances, vous pourrez...

This is different from some situations in English, where repetition can sometimes be reduced more easily.

French generally wants a clear subject before each conjugated verb.

It can mean either:

• one person in a formal way
• more than one person in a plural way

So this sentence could mean:

• speaking politely to one person: When you are on vacation...
• speaking to several people: When you are on vacation...

Only context tells you which one is intended.

Because dans la mer means in the sea, literally in the water.

• dans la mer = in the sea
• à la mer = at the seaside / by the sea

So:

• vous baigner dans la mer = swim/bathe in the sea
• aller à la mer = go to the seaside

This sentence is specifically about being in the water, so dans la mer is the natural choice.

French usually requires an article with nouns like mer.

So you say:

• dans la mer
• dans la piscine
• dans le lac

Even when speaking generally, French often keeps the article where English might seem less article-focused.

Here, la mer simply means the sea as the natural body of water being referred to.

They are very close in meaning.

• chaque matin = each morning / every morning
• tous les matins = every morning / all mornings

In many cases, either one would work.

Here, chaque matin sounds perfectly natural and emphasizes the repeated occurrence, one morning at a time.

So:

• vous pourrez vous baigner dans la mer chaque matin
• vous pourrez vous baigner dans la mer tous les matins

Both are possible, with only a very small nuance.

Yes. Lorsque also means when.

So you could say:

• Lorsque vous serez en vacances, vous pourrez vous baigner dans la mer chaque matin.

The meaning is basically the same. The difference is mostly style:

• quand = more common and everyday
• lorsque = a little more formal or written

In normal conversation, quand is usually the more natural choice.