If the shark and other predatory sea creatures are from an extraterrestrial world, how could they survive in the artificially salted oceans of earth?

They were re-adapted in the same way Creator re-adapted many, many, sea creatures to be salt-tolerant. This is not difficult for the advanced extraterrestrials who have mastered genetic manipulation for all kinds of benefits for themselves and they understand animal life similarly, and have done much tinkering with the species of their world to optimize them for various purposes and to weaponize them as well, for introduction into other worlds. This is a standard operating procedure for them and is part of their cultural way of life as a predatory species themselves.