haroukeihou de yoidome wo motte iku.

In this sentence, で marks the reason/cause.

• 波浪警報で = because of the high-wave warning / due to the high-wave warning

This is a common use of で with nouns, especially when talking about causes such as:

• weather
• accidents
• events
• public conditions

For example:

• 雨で試合が中止になった。
  The game was canceled because of rain.

So here, 波浪警報で explains why the speaker is taking motion-sickness medicine.

波浪警報 is read はろうけいほう.

It is an official weather/marine warning meaning a high wave warning. It refers to dangerous sea conditions with strong waves.

Breakdown:

• 波浪（はろう） = waves / sea waves
• 警報（けいほう） = warning / alarm

So this is not just a casual statement like the sea looks rough. It sounds like an official warning.

Yes. 酔い止め is read よいどめ, and in this kind of context it usually means medicine to prevent motion sickness.

Breakdown:

• 酔い（よい） = sickness/intoxication; in travel contexts, this often refers to motion sickness
• 止め（どめ） = stopper / something that prevents

So 酔い止め literally means something like anti-sickness or sickness-preventer, but in natural English here it is best understood as:

• motion-sickness medicine
• seasickness medicine

Because the sentence mentions a wave warning, the seasickness meaning is especially likely.

を marks 酔い止め as the direct object of the verb 持っていく.

So:

• 酔い止めを持っていく = take/bring motion-sickness medicine

The thing being taken is 酔い止め, so it gets を.

This is a very common question.

• 持っていく = take something from here to there
• 持ってくる = bring something to here or toward the speaker/listener’s location

In this sentence, 持っていく suggests the speaker is going somewhere and taking the medicine along to that destination.

So the image is:

• leaving the current place
• carrying the medicine to the place they are going

If it were 持ってくる, the direction would feel different: bringing it to the place of the speaker/listener.

It comes from 持つ in the て-form:

• 持つ → 持って
• 持っていく

Literally, it is hold/carry + go, but as a set expression it means:

• take along
• bring with you
• carry to another place

So yes, it is built from smaller parts, but learners should understand it as a very common compound-like expression with a clear directional meaning.

You may also see it written:

• 持っていく
• 持って行く

Both are normal.

Japanese often omits the subject when it is understood from context.

So 波浪警報で 酔い止めを 持っていく。 could mean:

• I’ll take motion-sickness medicine because of the high-wave warning.
• We’re taking motion-sickness medicine because of the high-wave warning.
• (Someone) is taking motion-sickness medicine because of the high-wave warning.

The exact subject depends on the situation.

This omission is completely normal in Japanese.

It is understandable, but by itself it can sound a little brief or note-like.

A fuller conversational version might be:

• 波浪警報だから、酔い止めを持っていく。
• 波浪警報が出ているから、酔い止めを持っていく。

These make the cause more explicitly conversational with だから or から.

The original sentence is still natural in contexts like:

• a memo
• a caption
• short spoken planning
• a list of actions/reasons

So it is not wrong; it is just somewhat compact.

No, not in this sentence.

で has several different uses in Japanese, such as:

• location of an action: 学校で勉強する = study at school
• means/method: バスで行く = go by bus
• cause/reason: 風邪で休む = be absent because of a cold

Here, with 波浪警報, the meaning is the cause/reason use.

So 波浪警報で does not mean at the wave warning. It means because of the wave warning.

Yes, very likely.

Because 波浪警報 is about dangerous sea waves, and 酔い止め is motion-sickness medicine, the sentence strongly suggests a situation like:

• going on a boat
• traveling by ferry
• being near rough seas

So the idea is probably that the rough sea conditions may cause seasickness, so the speaker plans to take medicine along.

That connection is not stated directly, but it is easy for a Japanese listener to infer.