Rashi on Leviticus 11:9/רש"י על ויקרא י״א:ט׳

1סנפיר — These are what it (the fish) swims with — FINS.

2קשקשת — These are the scales attached to it (see Chullin 59a) as it is said (I Samuel 17:5) "in a scaly coat of mail (שריון קשקשים) was he clothed".