Fish that have scales for part of their lives -- are likely to be kosher. But fish that have fins and no scales are not kosher. But if scales are all that matters, why doesn't the Torah just say scales? Why mention fins at all? The Gemara gives several answers. Also, the Gemara focuses on the language of the permitted and prohibited -- both of which forms are stated. Plus, eating that which grow in water is permitted, say, pits and ditches, but what about vessels? How did these creatures emerge from the water?!