Midrash su Levitico 11:9

1) (Vayikra 11:9): ("This you may eat of all that is in the waters: All that have fins and scales in the water, in the seas and in the rivers — those may you eat.") From its being written "you may eat" what has (fins and scales), I understand not to eat what does not have (fins and scales); and from its being written (Vayikra 11:11) not to eat what does not have, I understand that I may eat what does have. Why was this repeated? To make them subject to (transgression of) a positive commandment and a negative commandment. "You may eat of all that is in the waters.": For I might think that since (small fish found in basins [as opposed to fish found in running streams]) are permitted explicitly (even without the signs of cleanliness) and by implication (I might think that) just as explicitly they are permitted only in basins, so by implication they are permitted only in basins — Whence would I derive that even if they are found in holes, pits, or caves, he may bend down and drink from them without apprehension? From ("You may eat of all) that is in the waters."

2) (Vayikra 11:2) "This is the animal that you may eat": We are hereby taught that Moses took hold of each animal and showed it to Israel and said to them: This you may eat and this you may not eat. (Vayikra 11:9): "This you may eat of all that is in the waters" — This you may eat and this you may not eat. (Vayikra 11:9): "These shall you hold in detestation of the fowl" — These shall you hold in detestation and these shall you not hold in detestation. (Vayikra 11:29): "This to you is what is tamei among the creeping things" — This is tamei and this is not tamei.

2) "All that have fins and scales in the waters, in the seas, and in the rivers — those may you eat': "in the seas (yamim)": This is the Great Sea (the Mediterranean), as it is written (Bereshith 1:10): "And the (great) ingathering of the waters, He called yamim." "and in the nechalim": These are the rivers, as it is written (Bereshith 2:10): "And a river went out of Eden to water the garden." This tells me only of oceans and rivers, that flow (both) in the dry season and in the rainy season. Whence do I derive for inclusion other waters, and other rivers, that flow in the rainy season but not in the dry season, until you include water in holes? From (the repetition of) "in the waters" (Bereshith 11:8, Bereshith 11:9).