Il nuoto in piscina è rilassante.

Because nuoto here is a noun, not a verb. It means swimming as an activity or sport, and in Italian general activities are often expressed with the definite article.

So:

• Il nuoto è rilassante = Swimming is relaxing
• Il calcio è divertente = Football/Soccer is fun
• La lettura è utile = Reading is useful

In English, we often leave the article out, but Italian usually includes it in this kind of general statement.

Italian can express this idea in two ways:

• Il nuoto in piscina è rilassante
• Nuotare in piscina è rilassante

Both are possible, but they are slightly different in feel:

• il nuoto = swimming as an activity, practice, or sport
• nuotare = to swim / swimming as the action itself

So Il nuoto in piscina è rilassante sounds a bit more like pool swimming is relaxing, while Nuotare in piscina è rilassante sounds more like swimming in a pool is relaxing.

It means in the pool or sometimes more naturally at the pool, depending on context.

In this sentence, in piscina modifies nuoto and means pool swimming, as opposed to swimming:

• al mare = in the sea
• al lago = in a lake
• in piscina = in a swimming pool

So the phrase is not just about water in general; it specifically means swimming done in a pool.

In piscina is the normal generic expression for in/at the pool.

Italian often omits the article in common location expressions like this when speaking generally:

• andare in piscina = to go to the pool
• essere in piscina = to be at the pool

If you say nella piscina, that usually points to a specific pool:

• nuotare nella piscina dell'hotel = to swim in the hotel pool

So:

• in piscina = generic, usual expression
• nella piscina = in a particular pool

Because è is the verb essere in the he/she/it is form.

• è = is
• e = and

The accent is very important because it distinguishes the verb from the conjunction.

So:

• Il nuoto in piscina è rilassante = Swimming in the pool is relaxing
• Il nuoto in piscina e rilassante would be incorrect

Because rilassante means relaxing, while rilassato means relaxed.

That is a very common distinction in Italian:

• rilassante = something that causes relaxation
• rilassato = someone or something that feels relaxed

So:

• Il nuoto in piscina è rilassante = pool swimming is relaxing
• Sono rilassato = I am relaxed

A similar pair is:

• interessante = interesting
• interessato = interested

Yes, but in this case the form does not visibly change for masculine and feminine singular.

Nuoto is masculine singular, and rilassante is singular too. Adjectives ending in -ante often have:

• singular: rilassante
• plural: rilassanti

Examples:

• Il nuoto è rilassante
• La musica è rilassante
• Gli esercizi sono rilassanti
• Le attività sono rilassanti

So it does agree, but the singular form looks the same for masculine and feminine.

Yes. Il nuoto in piscina è rilassante is the most normal and neutral word order.

It follows a straightforward pattern:

• subject: Il nuoto in piscina
• verb: è
• adjective/complement: rilassante

You could change the order for emphasis, but that would sound less neutral. For example:

• È rilassante il nuoto in piscina

This is grammatical, but more marked and less common in everyday use.

They are related, but not the same.

• nuoto = swimming in general, the activity or sport
• nuotata = a swim, one swimming session

So:

• Il nuoto in piscina è rilassante = Swimming in a pool is relaxing
• Una nuotata in piscina è rilassante = A swim in a pool is relaxing

If you want to talk about swimming as a general activity, nuoto is the better choice here.

Yes. That is one of the reasons il nuoto is used.

Il nuoto in piscina can suggest:

• swimming as an activity
• swimming as exercise
• pool swimming as a sport or discipline

So the sentence can sound slightly broader than just the act of swimming right now. It can mean that the activity of pool swimming, in general, is relaxing.

Yes. That would mean Swimming is relaxing in a general sense.

Adding in piscina makes it more specific:

• Il nuoto è rilassante = Swimming is relaxing
• Il nuoto in piscina è rilassante = Swimming in a pool is relaxing

So the longer sentence narrows the meaning from swimming in general to pool swimming specifically.