General Information:

Moses continues speaking to the people of Israel.

Of these things that are in water you may eat

"You may eat these kinds of animals that live in the water"

the thin, flat part which the fish uses to move through the water

the small plates that cover the body of the fish

Something Yahweh says is unfit for his people to eat is spoken of as if it were physically unclean. (See: rc://en/ta/man/translate/figs-metaphor)