Learn Spanish (Spain): No creía que el crucero hubiera terminado tan mal. - I didn't believe that the cruise had ended so badly.

There are two things happening here. First, No creía que... (I didn't believe that...) is a negated belief in the past, which triggers the subjunctive mood. Second, the cruise ending happened before the past moment when you didn't believe it. To express a "past before a past" in the subjunctive, we use the Past Perfect Subjunctive: the imperfect subjunctive of the auxiliary verb haber (hubiera) plus the past participle (terminado).

Could I use hubiese terminado instead?

Yes! The imperfect subjunctive of haber has two valid forms: hubiera and hubiese. You can use them interchangeably, so No creía que el crucero hubiese terminado tan mal is completely correct and means the exact same thing.

If I wanted to say "I did believe that the cruise had ended so badly", would I still use the subjunctive?

No. Affirmative verbs of belief or thought (like creer and pensar) declare what you consider to be a fact, so they require the indicative mood. You would use the Past Perfect Indicative instead: Creía que el crucero había terminado tan mal. It is strictly the negation (No creía que...) that introduces doubt and triggers the subjunctive.

Why is it tan mal (so badly) instead of tanto mal?