Через иллюминатор было видно море, и мне сразу захотелось снова в отпуск.

Было видно is an impersonal construction meaning it was possible to see or simply you/I could see.

In Russian, видно can function like a predicative word meaning visible or able to be seen.
So:

• Через иллюминатор было видно море = The sea was visible through the porthole/window
• more naturally in English: Through the porthole, you could see the sea

The verb было is in the neuter singular past, which is very common in impersonal sentences in Russian.

Because море is not the direct object of a normal active verb here.

In было видно море, the noun names what was visible. In this kind of impersonal structure, Russian often keeps that noun in the nominative:

• Было видно море = The sea was visible
• Было слышно музыку is different, because слышно often takes the accusative more readily in modern usage

So here море is treated more like the thing that appears in view, not like a direct object being acted on.

Через usually means through when followed by the accusative.

So:

• через иллюминатор = through the porthole

This is a very literal spatial use: you are looking through something transparent or opening-like.

Other meanings of через exist too, such as across or in/after with time, but here it is simply through.

Иллюминатор means a porthole or a small window, especially on:

• a ship
• an airplane
• sometimes a spacecraft

In this sentence, it is probably the kind of window from which you can look out and see the sea.

Formally:

• dictionary form: иллюминатор
• here: иллюминатор after через, which requires the accusative
• since it is masculine inanimate, accusative singular = nominative singular, so the form does not change

Because захотелось comes from захотеться, which is an impersonal verb meaning something like:

• to feel like
• to suddenly want

The structure is:

• мне захотелось = I suddenly felt like... / I wanted...

Here мне is in the dative, which is very common in Russian for experiences, feelings, and states that happen to a person.

Compare:

• Я захотел поехать = I wanted to go
  more direct, personal
• Мне захотелось поехать = I felt like going / I got the urge to go
  more spontaneous, more like a feeling that arose

So мне сразу захотелось suggests an immediate emotional reaction.

Because this is another impersonal past-tense construction.

In Russian, impersonal past forms are often in the neuter singular, even when they refer to a person's feeling or experience:

• мне захотелось
• мне показалось
• мне вспомнилось

There is no grammatical subject like я controlling the verb, so the verb appears in the default neuter singular past form.

The -сь / -ся part is part of the verb хотеться / захотеться.

This is an aspect question.

• хотелось = imperfective
  suggests an ongoing desire or a general feeling of wanting
• захотелось = perfective
  suggests the desire arose, often suddenly

In this sentence:

• мне сразу захотелось = I immediately felt like...

The perfective verb fits well with сразу because it marks the beginning of the desire.

Сразу means right away, immediately, or at once.

So:

• мне сразу захотелось = I immediately felt like...

It emphasizes how quickly the sight of the sea produced that feeling.

Russian often leaves out a verb when the meaning is obvious.

Here:

• мне сразу захотелось снова в отпуск

literally feels like:

• I immediately felt like [going] on vacation again

The implied idea is something like:

• поехать в отпуск
• отправиться в отпуск

Russian can omit that verb because в отпуск already strongly suggests the idea of going.

So the sentence is natural and compact.

Because Russian uses the fixed expression:

• идти / поехать / отправиться в отпуск = to go on vacation / go on leave

The preposition в is the normal one with отпуск in this meaning.

Examples:

• Я скоро пойду в отпуск. = I’ll go on vacation soon.
• Он уехал в отпуск. = He went on vacation.

Снова means again.

In this sentence:

• снова в отпуск = on vacation again

Very often, снова and опять can both mean again. But learners are often told this tendency:

• снова can sound a bit more neutral or literary
• опять can sometimes sound more emotional, and depending on context, even slightly annoyed: again!

Here снова is a natural, neutral choice.

Yes, Russian word order is flexible, but the original order sounds natural and expressive.

Original:

• Через иллюминатор было видно море, и мне сразу захотелось снова в отпуск.

This order works well because:

1. it starts with the visual scene
2. then gives the emotional reaction

You could rearrange parts, but the emphasis would change. For example:

• Мне сразу захотелось снова в отпуск, когда через иллюминатор было видно море.

This is possible, but it sounds less smooth and less natural in this context.

It is mostly neutral natural Russian.

Nothing in it is slangy or highly formal. It would fit in:

• a story
• a memoir
• conversation
• travel writing

A few words, like иллюминатор, are a bit specific in meaning, but not stylistically unusual.

Yes. That is one of the most useful things to notice in this sentence.

There are two impersonal structures:

1. было видно море
   = the sea was visible / you could see the sea

2. мне захотелось снова в отпуск
   = I felt like going on vacation again

This is very typical Russian style: instead of always saying I saw or I wanted, Russian often uses structures that present things as:

• something being visible
• a desire arising in someone

That can sound more natural and idiomatic in Russian.