umi he iku hi ha, hiyakedome wo motte ikanai wake ni ha ikanai.

In Japanese, a verb can directly modify a noun.

So 海へ行く日 is:

• 海へ行く = going to the beach
• 日 = day

Together, it means the day(s) when someone goes to the beach.

You do not put の between a verb and the noun it modifies. This is one of the most basic ways Japanese makes relative clauses.

Compare:

• 本を読む人 = a person who reads books
• 日本へ行く日 = the day when one goes to Japan

So 海へ行く日 is completely natural Japanese structure.

The は marks 海へ行く日 as the topic of the sentence.

So the structure is basically:

• 海へ行く日は = as for days when I go to the beach...

Then the rest of the sentence says what is true about those days.

This kind of は often gives a general or contrastive feeling, like:

• on beach-going days, this is what applies
• when it is a day to go to the beach, this is the situation

It is not just identifying a day; it is setting up a category of days and then commenting on them.

Yes, 海に行く would also be natural.

Both に and へ can mark a destination with verbs like 行く.

A rough difference:

• に focuses more on the destination as a target
• へ focuses more on the direction toward that destination

In many everyday sentences with 行く, they are both possible and the difference is small.

So:

• 海へ行く日
• 海に行く日

are both fine. The version with へ may sound just a little more directional, but this is not a major meaning change here.

持っていく means to take something along or to bring something to a place away from where you are now.

It is built from:

• 持つ = to hold, to carry
• 持っていく = to take/carry something with you to another place

So in this sentence, it means taking sunscreen along to the beach.

A helpful comparison:

• 持っていく = take it there
• 持ってくる = bring it here

In English, take and bring often depend on viewpoint, and Japanese works similarly.

This is a fixed grammar pattern:

• Vないわけにはいかない

It means:

• cannot reasonably avoid doing V
• have no choice but to do V
• must do V

So:

• 持っていかない = not take it
• 持っていかないわけにはいかない = it will not do to not take it
• natural English: I have to take it

The logic is:

• not doing it is not acceptable
• therefore, doing it is necessary

That is why it looks double-negative in Japanese, even though the overall meaning is positive obligation.

Historically it is the same verb 行く, but in this pattern it is functioning as part of an idiomatic grammar expression.

So in:

• 海へ行く日 — 行く is literal: to go
• わけにはいかない — いかない is part of the set phrase meaning it cannot be helped / it is not possible / it is not acceptable

In other words, in わけにはいかない, you usually should not try to interpret いかない as a literal physical go. It is better to learn the whole pattern as one unit.

By itself, わけ can mean something like:

• reason
• circumstance
• situation
• logic behind something

But in this sentence, わけにはいかない is a fixed expression, so わけ is not being used in a simple standalone dictionary sense.

It is better to understand the whole chunk:

• Vないわけにはいかない = cannot avoid doing V

So while わけ contributes the idea of circumstances / reasoning, learners usually get the best results by memorizing the full grammar pattern rather than translating わけ word by word.

All of them can often be translated as have to, but the nuance is a little different.

• Vなければならない / Vなくてはいけない
  More direct statement of obligation or necessity

• Vないわけにはいかない
  Strong sense that, given the circumstances, you cannot very well choose not to do it

So 持っていかないわけにはいかない suggests something like:

• considering the situation, not bringing sunscreen is not really an option

This can sound a bit more situational, practical, or reluctant than a plain must.

It often implies:

• social expectations
• common sense
• practical necessity
• reluctance, but no real choice

That is very common in grammar patterns.

Even though わけ can be written as 訳, Japanese often uses kana for very common grammatical or idiomatic expressions because it looks lighter and more natural.

So:

• わけにはいかない
• 訳にはいかない

are both possible, but the kana version is extremely common.

Many textbooks also prefer kana for grammar patterns because it makes it easier to recognize the structure as a grammar item rather than as a literal noun.

It can be understood either way depending on context, but without extra context it often has a general/habitual feel:

• on days when I go to the beach
• whenever I go to the beach

That comes from the combination of:

• a plain verb before a noun: 海へ行く日
• the topic marker は

So it sounds like the speaker is talking about a general rule or repeated situation, not necessarily just one single calendar day.

If the speaker meant one very specific day, context would usually make that clear.

Yes, the subject is omitted, which is very normal in Japanese.

The sentence does not explicitly say:

• I
• you
• we
• someone

Japanese often leaves that unstated when it is clear from context.

In many real situations, this sentence would naturally be understood as something like:

• I have to bring sunscreen
• or more generally, you have to bring sunscreen on beach days

The exact subject depends on who is speaking and what the conversation is about. The grammar itself does not force one single subject.