فيه ميه في النهر.

A common Egyptian pronunciation is:

fīh mayya fin-nahr

A few notes:

• فيه = fīh
  
  • long ee sound
  • the h can be light in fast speech
• ميه = mayya or meyya
  
  • the everyday Egyptian word for water
• في النهر is often pronounced fin-nahr
  
  • the l of ال is not pronounced here because of the ن

So the whole sentence may sound very close to feeh mayya fin-nahr.

Here, فيه means there is / there are.

It is very common in Egyptian Arabic for talking about existence:

• فيه ميه = there is water
• فيه ناس = there are people

So in this sentence, فيه is not being used as a normal preposition + pronoun combination. It is functioning as an existential expression: there is.

Because the two parts are doing different jobs:

• فيه = there is / there are
• في النهر = in the river

So although both contain في, they are not the same grammatical element in the sentence.

A very literal breakdown is:

• فيه = there is
• ميه = water
• في النهر = in the river

So: There is water in the river.

Because Arabic does not normally use a present-tense verb to be the way English does.

In English, you need:

• There is water...

In Egyptian Arabic, the idea is expressed with فيه for existence, so no extra word for is is needed.

This is very normal in Arabic. In many present-tense sentences, Arabic simply does not use a word equivalent to English is/am/are.

Yes. In everyday Egyptian Arabic, ميه is the usual word for water.

You may also see it written as:

• ميه
• مية
• مَيّه

These are spelling variants used in less formal writing.

In Modern Standard Arabic, the word is ماء, but in Egyptian speech people normally say ميه.

Because النهر means the river, while نهر means a river.

So:

• في النهر = in the river
• في نهر = in a river

If the meaning shown to the learner is in the river, then النهر is exactly what you would expect.

It is written النهر, but in pronunciation the ل of ال disappears because ن is a sun letter.

So instead of saying el-nahr, you say something like:

en-nahr or in-nahr

After في, it often becomes:

fin-nahr

So even though the spelling has ال, the pronunciation does not keep a clear l sound here.

In Egyptian Arabic, فيه is commonly used for both there is and there are.

For example:

• فيه كتاب = there is a book
• فيه كتب = there are books
• فيه ناس = there are people

So unlike English, Egyptian Arabic usually does not change this word based on singular vs. plural.

In your sentence, ميه is an uncountable noun anyway, so فيه is exactly what you would expect.

Yes, sometimes.

The sentence فيه ميه في النهر is a very natural existential pattern:
There is water in the river.

You may also hear:

في النهر ميه

This puts the location first, something like:

In the river, there is water.

The basic meaning stays very close, but the emphasis can shift a little depending on context.

A very common Egyptian negative is:

مافيش ميه في النهر

This means:

There isn't any water in the river.

So the pattern is:

• فيه = there is / there are
• مافيش = there isn't / there aren't

This is one of the most useful everyday patterns in Egyptian Arabic.