حارة السباحة تحت مراقبة المنقذ وقت السباح يغطس.

That is a very common confusion. حارة can mean neighborhood or alley-quarter in many everyday contexts, but in sports or pool contexts it can also mean lane.

So here:

• حارة السباحة = the swimming lane / the pool lane

It does not mean the swimming neighborhood.

Because this is an iḍāfa (construct phrase): حارة + السباحة.

In an iḍāfa:

• the first noun usually does not take ال
• the second noun determines whether the whole phrase is definite

So:

• حارة السباحة = the swimming lane

Since السباحة is definite, the whole phrase is definite. This is a very common Arabic pattern.

Because Arabic normally leaves out the present-tense verb to be.

So the Arabic structure is literally something like:

• the swimming lane under the lifeguard's supervision

But natural English needs is:

• The swimming lane is under the lifeguard's supervision

This kind of sentence is very common in Arabic.

Literally, it means:

• under the monitoring/watch of the lifeguard

In smoother English, that becomes:

• under the lifeguard's supervision
• under the lifeguard's watch

A useful nuance:

• مراقبة = monitoring, watching
• إشراف = supervision

So if you want a slightly more natural phrase for under the lifeguard's supervision, many speakers would prefer:

• تحت إشراف المنقذ

Yes, if you take it completely by itself, it can be a little ambiguous.

Arabic verbal nouns like مراقبة can sometimes allow more than one interpretation. But in this sentence, the intended meaning is clearly:

• the lifeguard is the one doing the monitoring

If you want to make that clearer and more natural, تحت إشراف المنقذ is often a better choice.

Literally, وقت means time. But here it introduces a time clause, so in English it works like:

• when
• while

So:

• وقت السباح يغطس = when/while the swimmer dives / is diving

In everyday Levantine, speakers often use:

• لما
• وقت ما

more naturally than bare وقت in this kind of sentence.

Because they are different words:

• السباحة = swimming (the activity)
• السبّاح = the swimmer

In normal Arabic writing, short vowels and the shadda are usually omitted, so السباح on the page may not show you that it is really السَّبّاح with a doubled b sound. Learners often have to work that out from context.

يغطس means:

• he dives
• he plunges
• he goes underwater

Depending on context, English might translate it as:

• dives
• is diving
• goes under the water

So it has the idea of entering or moving down into the water.

Arabic uses the present tense for several things, including:

• actions happening now
• repeated actions
• general statements

So وقت السباح يغطس could mean:

• when the swimmer dives
• while the swimmer is diving

The best English tense depends on the context, not just the Arabic form by itself.

It is understandable, but it sounds somewhat formal or Standard Arabic-like rather than strongly conversational Levantine.

A more everyday Levantine-style version might be:

• حارة السباحة تحت إشراف المنقذ لما السباح يغطس
• المنقذ عم يراقب حارة السباحة لما السباح يغطس

So the original sentence is useful for learning, but a native Levantine speaker might phrase it a bit differently in casual speech.

Because of the sun-letter / moon-letter rule.

• س is a sun letter, so السباحة is pronounced roughly es-sbāḥa, and السبّاح as es-sabbāḥ. The l of ال assimilates to the s sound.
• م is a moon letter, so المنقذ keeps the l sound.

So the spelling stays the same, but the pronunciation changes depending on the first consonant of the word after ال.