seimei ni ha mizu ga hituyou desu.

It is read:

せいめい には みず が ひつよう です。

• 生命 = せいめい
• 水 = みず
• 必要 = ひつよう

A natural rhythm is:

せいめいには / みずが / ひつようです

生命 means life in a general, biological, or abstract sense.

It is similar to 命, but the nuance is different:

• 生命 = life as a phenomenon, biological life, a formal/scientific word
• 命 = life in a more personal or emotional sense, like one’s life

So in a statement like this, 生命 sounds more natural because it is talking about life in general, not a particular person's life.

The particle に shows the thing or person for which something is necessary.

So:

• 生命に必要です = is necessary for life

This is a very common pattern:

A に B が必要です
= B is necessary for A
= A needs B

So here:

• 生命に = for life
• 水が必要です = water is necessary

には is に + は.

The は adds a topic or slight contrast nuance. It can sound like:

• as for life
• for life, at least
• when it comes to life

So:

• 生命に水が必要です = Water is necessary for life.
• 生命には水が必要です = As for life, water is necessary.

In many sentences like this, には sounds very natural and helps set up the topic.

Because 必要 is not a regular verb.

It is a word meaning necessary, and with 必要, the thing that is needed is usually marked with が:

• 水が必要です = water is necessary / water is needed

So the common pattern is:

A に B が必要です

Not:

A に B を必要です ❌

If you want to use を, you need a different structure, such as the verb-like expression:

• 水を必要とする = to require water

But in this sentence, 水が必要です is the standard pattern.

必要 is a na-adjective (also called an adjectival noun).

That means it can work in different ways:

• 必要です = is necessary
• 必要な水 = necessary water
• 水が必要だ = water is necessary

In this sentence, 必要です is the polite predicate.

So the sentence ends politely and neutrally.

Not exactly.

The topic is 生命には, but the thing marked by が is 水, so grammatically 水 is the thing being presented as necessary.

A very literal breakdown is:

• 生命には = as for life / for life
• 水が = water
• 必要です = is necessary

So the sentence is closer to:

For life, water is necessary.

Japanese often separates topic and grammatical subject, so the topic is not always the same as the subject.

Yes, you can.

• 生命に水が必要です = straightforward statement
• 生命には水が必要です = adds topic emphasis or a slight contrast

The version with は often sounds a little more like:

• when it comes to life
• for life, specifically

So both are possible, but には gives a slightly more marked, topic-setting feel.

Yes. Japanese word order is fairly flexible as long as the particles stay correct.

For example:

• 生命には水が必要です
• 水は生命に必要です

Both are possible, but the focus changes.

• 生命には水が必要です highlights life as the topic
• 水は生命に必要です highlights water as the topic

So the original sentence is choosing to frame the statement around life.

です makes the sentence polite.

So:

• 生命には水が必要です = polite
• 生命には水が必要だ = plain

The meaning is basically the same, but です is the form you will often see in textbooks and neutral polite writing.

Literally, it is closer to life needs water or water is necessary for life.

生命 means life, not specifically living things.

If you wanted to say living things need water, you would usually use a different word, such as:

• 生き物
• 生物

Still, depending on context, English may translate the idea more naturally as living things need water, even though the Japanese word here is specifically 生命.