Learn Spanish (Latin America): Como hay una alarma, no podemos sumergirnos. - Since there is an alarm, we cannot submerge.

When you state the reason at the very beginning of a sentence, Spanish almost always uses como. If you wanted to use porque, you would normally put the reason in the second half of the sentence: No podemos sumergirnos porque hay una alarma.

Why is hay in the indicative mood? Don't we sometimes use the subjunctive with como?

When como is used to explain the cause of something and states a known fact, it always takes the indicative mood (like hay). You would only use the subjunctive after como if you were setting a condition or giving a warning, where it translates closer to if or provided that.

Why is nos attached to the end of sumergir? Could it go anywhere else in the sentence?

Yes, you have two options! When a conjugated verb (podemos) is followed by an infinitive (sumergir), the pronoun can be attached to the end of the infinitive, as in sumergirnos. Alternatively, you can place it right before the conjugated verb: no nos podemos sumergir. Both options are perfectly natural.

Is sumergirnos acting as a reflexive verb here?