Revisa la junta de la puerta, no sea que salga agua.

Yes. Revisa is the tú imperative of revisar, so it means check / inspect when speaking informally to one person.

• Revisa la junta... = Check the seal...
• If you were speaking formally to one person, it would be Revise la junta...
• To more than one person in Spain, it would usually be Revisad la junta...

Also, revisar often means to check, inspect, look over, not just to revise in the English study sense. So this is a very common false friend for English speakers.

Here, la junta means the seal, gasket, or rubber lining around the door.

This is one of those words with several meanings in Spanish. Depending on context, junta can also mean:

• meeting
• committee / board
• joint
• seal / gasket

In this sentence, because it is la junta de la puerta and water might come out, the meaning is clearly door seal.

Because de la puerta specifies which seal we are talking about: the seal of the door or more naturally in English, the door seal.

Spanish often uses de + noun where English prefers a compound noun:

• la puerta del coche = the car door
• la tapa de la caja = the box lid
• la junta de la puerta = the door seal

So this is a very normal Spanish structure.

No sea que is a fixed expression meaning something like:

• in case
• lest
• for fear that

So:

• Revisa la junta de la puerta, no sea que salga agua. = Check the door seal, in case water comes out / so that water doesn’t leak out.

It expresses a precaution against something undesirable.

It can sound a bit more formal or careful than some other ways of saying the same idea.

Because sea is the subjunctive form of ser, not the normal present indicative es.

The expression no sea que requires the subjunctive because it introduces something feared, uncertain, or hypothetical.

Compare:

• Es verdad. = It is true. → indicative
• No sea que... = In case / lest... → subjunctive structure

Here, sea does not mean is in a literal, standalone way. It is part of the set phrase no sea que.

Because after no sea que, Spanish uses the subjunctive, so the verb salir becomes salga.

• sale = indicative, it comes out / it leaves
• salga = subjunctive, it might come out / should come out

This is one of the key patterns learners need to get used to:

• expressions of fear, prevention, possibility, or purpose often trigger the subjunctive

So:

• no sea que salga agua = in case water comes out / lest water leak out

Yes, salga comes from salir.

In the present subjunctive, salir is irregular:

• yo: salga
• tú: salgas
• él/ella/usted: salga
• nosotros: salgamos
• vosotros: salgáis
• ellos/ustedes: salgan

It looks different because many Spanish verbs have irregular first-person present forms, and the present subjunctive is often built from that stem:

• salgo → salga
• tengo → tenga
• hago → haga

Because agua here is being used as an uncountable noun in a general sense: water.

Spanish often omits the article with uncountable nouns when speaking generally, especially after verbs:

• Sale agua. = Water comes out.
• Hay agua. = There is water.

You could sometimes see an article in other contexts, but here salga agua is completely natural.

Yes, and that would also be very natural, but the nuance is slightly different.

• Revisa la junta de la puerta, no sea que salga agua. = Check the door seal, in case water comes out.

  • focus on a possible bad outcome you want to avoid

• Revisa la junta de la puerta para que no salga agua. = Check the door seal so that water doesn’t come out.

  • focus on purpose

Both are correct, but no sea que sounds more like a warning or precaution.

Yes, it is natural and understandable in Spain.

A few notes:

• Revisa is normal for an informal instruction to one person.
• No sea que is perfectly correct and used in Spain, though it can sound a little more careful or formal than simpler alternatives.
• In casual speech, someone might also say:
  • Revisa la junta de la puerta, a ver si va a salir agua.
  • Revisa la junta de la puerta, por si sale agua.
  • Revisa la junta de la puerta para que no salga agua.

So the original sentence is good Spanish, especially for instructions, warnings, or practical advice.

A useful pattern to remember is:

Imperative + precaution expression + subjunctive

In this sentence:

• Revisa = imperative
• no sea que = precaution / fear expression
• salga = subjunctive

So the full pattern is:

• Do X, in case Y happens
• Haz X, no sea que pase Y

Examples:

• Llévate un paraguas, no sea que llueva. = Take an umbrella, in case it rains.

• Cierra bien la puerta, no sea que entre frío. = Close the door properly, in case cold air gets in.

That is probably the most important takeaway from this sentence.