Learn Hardcore Spanish (Latin America): Si la marea subiera más, cerrarían la playa por seguridad. - If the tide rose further, they would close the beach for safety.

We use subiera because this is an unreal or hypothetical “if” clause (la protasis) referring to something that isn’t happening right now. In Spanish, for these second-type conditionals you pair the imperfect subjunctive (here, subiera) in the si-clause with the conditional in the main clause.

sube would be present indicative, used for real or possible situations (“If the tide rises, they close the beach”).
subiría would be conditional, but you don’t put the conditional in the si-clause; it goes in the main clause.

Why do we use cerrarían instead of cerrarán or cierran?

Since the si-clause is in the imperfect subjunctive (subiera), the main clause needs the conditional (cerrarían). This construction expresses “they would close” — a hypothetical result.

cerrarán is simple future (“they will close”), used with a real or likely condition (present indicative).
cierran is present indicative (“they close”), also for real situations, not for hypothetical ones.

What type of conditional sentence is this in Spanish, and how does it compare to English?

This is the “second conditional” or “hypothetical present-future” conditional. In English you’d say “If the tide rose any more, they would close the beach for safety.” Both languages use a past-style form in the if-clause (imperfect subjunctive in Spanish, simple past in English) and a “would” construction in the main clause.

Could you rewrite this sentence to express a real possibility instead of a hypothetical one?

Yes. To express a real possibility you use present indicative in the si-clause and future in the main clause:
Si la marea sube más, cerrarán la playa por seguridad.
Here you’re saying “If the tide rises more (and this is likely), they will close the beach.”

What does por seguridad mean, and can we say it differently?

Por seguridad means “for safety” or “for security reasons.” It explains the motive: they close the beach to keep people safe. You could also say por motivos de seguridad (“for reasons of security”), but it’s a bit longer.

What if I wanted to talk about a past hypothetical? How would I say “If the tide had risen more, they would have closed the beach”?

You’d use the pluperfect subjunctive in the si-clause and the conditional perfect in the main clause:
Si la marea hubiera subido más, habrían cerrado la playa por seguridad.