Are all seafood permissible, and is there a religious text that prohibits eating whales and marine mammals such as seals and turtles? And what is the criterion for distinguishing between permissible and impermissible marine creatures?

The basic principle regarding sea animals that live solely in water is that they are permissible (halal), whether alive or dead, based on the Almighty's saying: (Lawful to you is game from the sea and its food) and the Prophet's (peace be upon him) saying about the sea: (Its water is purifying, and its dead are lawful).

Eating whale is permissible because it is a type of fish, and the fact that it gives birth rather than lays eggs does not affect its legal ruling.

Shark meat is permissible due to the aforementioned evidence. Its predatory nature does not prevent this, because the prohibition of every carnivore with fangs applies specifically to land carnivores, not sea carnivores.