Albert Barnes Commentary Leviticus 11:9

"These may ye eat of all that are in the waters: whatsoever hath fins and scales in the waters, in the seas, and in the rivers, that may ye eat." — Leviticus 11:9 (ASV)

Any fish from either salt water or fresh water could be eaten if it had both scales and fins, but no other creature that lives in the waters was permitted. Shellfish of all kinds, whether mollusks or crustaceans, and cetaceous animals, were therefore prohibited, as well as fish that appear to have no scales, like the eel; probably because they were considered unwholesome and, under certain circumstances, found to be so.