Mi amiga no quería meterse en el agua, pero acabó por ponerse el bañador al ver que todas ya nadábamos.

Meterse en el agua is a very common everyday way to say to get into the water. It sounds natural for situations like going into the sea, a pool, or a river.

Entrar en el agua is also possible, but meterse often feels more idiomatic and physical, like actually putting yourself into the water. In casual Spanish, especially in speech, meterse en el agua is extremely common.

This is a classic Spanish point. Agua is grammatically feminine, but in the singular it usually takes el instead of la when the article comes directly before it: el agua, el águila, el aula.

That happens because the word begins with a stressed a sound, and Spanish avoids la agua because it sounds awkward.

But the word is still feminine, so adjectives stay feminine:

• el agua fría
• el agua limpia

In your sentence, en el agua follows that same rule.

No quería uses the imperfect and describes an ongoing state, attitude, or reluctance: she didn’t want to get in the water.

This fits the idea that, for a while, she was unwilling.

If you said no quiso, it would sound more like a definite refusal at a particular moment: she refused / she didn’t want to on that specific occasion.

So:

• no quería = ongoing feeling or attitude
• no quiso = one completed act of refusal

Acabó por ponerse el bañador means she ended up putting on her swimsuit or she finally put on her swimsuit.

The structure acabar por + infinitive often expresses that, after hesitation, resistance, or a process, something finally happened.

So here the idea is:

• first, she did not want to get into the water
• then, after seeing everyone else swimming, she eventually gave in

It is not just about the literal end of an action. It strongly suggests in the end / eventually / after all.

Ponerse means to put on oneself, so it is the normal verb for clothing.

Compare:

• poner algo = to put something somewhere
• ponerse algo = to put something on

So:

• puso el libro en la mesa = she put the book on the table
• se puso el bañador = she put on her swimsuit

With clothes, Spanish very often uses reflexive verbs:

• ponerse = to put on
• quitarse = to take off
• vestirse = to get dressed

Yes, bañador is very common in Spain for swimsuit.

Regional vocabulary varies:

• In Spain, bañador is standard.
• In many parts of Latin America, people may say traje de baño.
• For a bikini, Spanish speakers might say bikini or biquini, depending on the region.

So if you are learning Spanish from Spain, bañador is exactly the kind of word you should expect.

Al ver que means when she saw that or upon seeing that.

This is the structure:

• al + infinitive

It often expresses when, upon, or on doing something.

Examples:

• Al llegar, me llamó. = When she arrived, she called me.
• Al verlo, sonrió. = On seeing him, she smiled.

So in your sentence:

• al ver que todas ya nadábamos = when she saw that we were all already swimming

Because the group being referred to is feminine.

The sentence begins with Mi amiga, and later todas shows that the speaker is talking about a group of females: all of us girls/women.

Also, the verb nadábamos is we were swimming, so todas includes the speaker as part of that female group.

If the group were masculine or mixed, Spanish would normally use todos.

Nadábamos is the imperfect, and here it describes a background action already in progress: we were swimming.

That is why it works so well after al ver que:

• she saw that we were already in the middle of swimming

Compare:

• nadábamos = we were swimming / we used to swim
• nadamos = we swam
• estábamos nadando = we were swimming

In this sentence, nadábamos and estábamos nadando are both possible, but nadábamos is often more natural and elegant in narration. Spanish uses the imperfect very often where English prefers the progressive form.

Ya means already here.

It adds the idea that, by that moment, the rest of us were already in the water and swimming. That helps explain why her friend changed her mind.

Without ya, the sentence would still make sense, but ya strengthens the contrast:

• she still did not want to get in
• meanwhile, we were already swimming

The subject is mi amiga.

The sentence has one main subject at the beginning:

• Mi amiga no quería... pero acabó...

So both quería and acabó refer to my friend.

Then inside the clause al ver que todas ya nadábamos, the subject changes:

• todas... nadábamos = all of us were swimming

So the sentence switches from she to we.

Yes, you may hear acabó poniéndose el bañador, and it can also mean she ended up putting on her swimsuit.

But acabó por + infinitive is a very common fixed pattern for eventually ending up doing something, especially in written or narrative Spanish.

So:

• acabó por ponerse el bañador = very clear, idiomatic, and slightly more formal or narrative
• acabó poniéndose el bañador = also possible, often more conversational

In your sentence, acabó por ponerse is an especially neat way to show that she finally gave in.

Because each past form does a different job:

• quería: imperfect
  Describes her ongoing reluctance or state of mind.

• acabó: preterite
  Marks the completed event that moved the story forward: she finally put on the swimsuit.

• nadábamos: imperfect
  Describes the background situation that she noticed: we were already swimming.

So the sentence uses:

• imperfect for background, ongoing states, and situations
• preterite for the key completed action

That contrast is very typical in Spanish storytelling.