TT #122 · Parshas Shemini · Aliyah 6 · Passuk 10
Chabad.org — Vayikra Chapter 11 with Rashi
English: But anything in the seas or streams that has no fins and scales among all the swarming things and all the living creatures in the water is an abomination for you.
Sifra (Toras Kohanim), Shemini, Section 3.
Sefaria — Sifra, Shemini, Section 3
נֶפֶשׁ הַחַיָּה. חַיָּה זוֹ חַיַּת הַיָּם, הַנֶּפֶשׁ לְהָבִיא אֶת הַסִּילונית, יָכוֹל תְּהֵא מְטַמָּא בְּאֹהֶל, תַּלְמוּד לוֹמַר זֹאת
Category distinction — “Living creature” includes a sea animal and “soul” adds the particular aquatic creature identified by the derivation; the verse's wording limits the resulting impurity rule.
The phrase includes sea creatures and the siren-like creature in the derashah while excluding it from corpse-tent impurity.