"We swim out of the sea: the shark is coming!"
Translation:Kiúszunk a tengerből: jön a cápa!
...A cápa jön Is also ok? Perhaps more emphasis on the shark. Or must it be jön a cápa?
It's better to put the emphasis on the verb. You can say "a cápa jön" as well, if you stress "jön" in speech, but it's better to put the verb in front. "A cápa jön", without any stress on the verb, expresses that it's the shark that is coming, and not something else, but that wouldn't be important in a context like this.