Δεν κάνω βουτιά στα βαθιά, γιατί προτιμώ τα ρηχά νερά κοντά στην ακτή.

Literally, it means I do/make a dive, but in natural English it is usually I dive, I take a dive, or I go for a dip.

In this sentence, Δεν κάνω βουτιά στα βαθιά means something like:

• I don’t dive into the deep water
• I don’t go into the deep end

Greek often uses κάνω + noun where English prefers a simple verb.

Examples:

• κάνω βόλτα = I take a walk
• κάνω μπάνιο = I bathe / go swimming
• κάνω βουτιά = I dive / take a dip

Because δεν is the normal negation for statements in the indicative:

• κάνω = I do / I dive
• δεν κάνω = I do not / I don’t dive

μη(ν) is used mainly for:

• commands: Μην κάνεις βουτιά! = Don’t dive!
• subjunctive structures: να μην κάνω = not to do / that I not do

So here, since this is a regular statement, δεν is the correct negative word.

Because κάνω βουτιά is a very common Greek pattern: κάνω + bare noun to describe an activity.

So:

• κάνω βουτιά = I dive / I take a dip
• κάνω μια βουτιά = I take one dive / a quick dip

Without an article, the phrase sounds more like a general activity.
If you add μια, it becomes more specific or countable.

Compare:

• Δεν κάνω βουτιά = I don’t dive / I’m not diving
• Δεν κάνω μια βουτιά = I’m not taking a dive / a dip

στα is the contraction of σε + τα.

So:

• σε = in, at, to
• τα = the
• στα = in the / at the / to the

In Greek, σε is very flexible and can cover meanings that English separates into in, at, and to.

So στα βαθιά can mean:

• in the deep water
• into the deep water
• at the deep end

The exact English translation depends on context.

Here βαθιά is a neuter plural adjective being used almost like a noun.

Literally, it means the deep ones/things, but the understood idea is:

• τα βαθιά (νερά) = the deep waters / the deep part

Greek often does this with adjectives when the noun is obvious from context.

So:

• στα βαθιά = in/into the deep waters, in/into the deep end

This is very natural Greek.

Because it agrees with an implied neuter plural noun, usually νερά.

So the full idea is:

• στα βαθιά (νερά) = in the deep waters
• τα ρηχά νερά = the shallow waters

Neuter plural is very common in expressions about areas, places, conditions, or abstract categories.

That is why you see:

• βαθιά
• ρηχά
• νερά

all in neuter plural form.

Yes, γιατί can mean both why and because.

Here it means because, since it introduces the reason:

• Δεν κάνω βουτιά στα βαθιά, γιατί προτιμώ...
• I don’t dive into the deep water, because I prefer...

As a question word, it would mean why:

• Γιατί δεν κάνεις βουτιά στα βαθιά;
• Why don’t you dive into the deep water?

So the meaning comes from context.

Greek uses the definite article much more often than English does.

So where English says:

• I prefer shallow waters

Greek naturally says:

• Προτιμώ τα ρηχά νερά

The article here refers to a general category, not necessarily a specific set of waters right in front of you.

This is very common in Greek:

• Μου αρέσει ο καφές = I like coffee
• Προτιμώ το τρένο = I prefer the train
• Προτιμώ τα ρηχά νερά = I prefer shallow waters

Greek often uses the plural νερά where English may use singular water.

So:

• ρηχά νερά = shallow waters / shallow water
• βαθιά νερά = deep waters / deep water

The plural is especially common when talking about:

• sea conditions
• swimming areas
• different depths
• bodies of water in a descriptive way

Other common examples:

• ήρεμα νερά = calm waters
• κρύα νερά = cold waters
• καθαρά νερά = clear waters

στην is the contraction of σε + την:

• σε = in / at / to
• την = the
• στην = in the / at the / to the

So:

• κοντά στην ακτή = near the coast / near the shore

ακτή is a feminine noun, and after σε in Modern Greek you normally use the accusative.

So the structure is:

• κοντά σε + accusative
• κοντά στην ακτή

Very similar examples:

• κοντά στο σπίτι = near the house
• κοντά στο λιμάνι = near the port

It is in the accusative, because it is the direct object of προτιμώ.

Breakdown:

• προτιμώ = I prefer
• τι προτιμώ; = what do I prefer?
• τα ρηχά νερά = the shallow waters

So:

• article: τα
• adjective: ρηχά
• noun: νερά

All are neuter plural accusative.
For neuter plural, nominative and accusative often look the same, so the form does not change here.

It can be both.

Literally:

• πέφτω στα βαθιά = I go into the deep water / deep end

Figuratively, στα βαθιά can mean something like:

• into the deep end
• into a difficult situation without preparation

For example:

• Τον έριξαν στα βαθιά από την πρώτη μέρα στη δουλειά.
• They threw him in at the deep end from the first day at work.

In your sentence, the meaning is clearly literal because of ρηχά νερά and κοντά στην ακτή.

Yes, absolutely. That would also be natural Greek.

Compare:

• Δεν κάνω βουτιά στα βαθιά
  = I don’t take a dive into the deep / I don’t dive into the deep water

• Δεν βουτάω στα βαθιά
  = I don’t dive into the deep water

The version with βουτάω uses a simple verb and sounds a bit more direct.
The version with κάνω βουτιά uses a very common light verb + noun structure.

Both are correct and natural.

Greek word order is fairly flexible, although the original sentence is the most neutral and natural:

• Δεν κάνω βουτιά στα βαθιά, γιατί προτιμώ τα ρηχά νερά κοντά στην ακτή.

You could rearrange parts for emphasis, for example:

• Τα ρηχά νερά κοντά στην ακτή προτιμώ, γι’ αυτό δεν κάνω βουτιά στα βαθιά.
• Γιατί προτιμώ τα ρηχά νερά κοντά στην ακτή, δεν κάνω βουτιά στα βαθιά.

These are grammatical, but they sound more marked or literary.
For a learner, the original order is the best model to follow.