From Rabbi Chaim Goldberg of the OU:
Q: Besides shellfish, which are some common non-kosher fish to be aware of?
A: Catfish – (family Ictaluridae) lack scales entirely. Interesting for the kosher consumer to note, non-kosher catfish is reported to have a similar taste to the increasingly popular (and kosher variety of) tilapia. Catfish and tilapia fillets look almost identical, though catfish is notably cheaper. It is quite possible that an unscrupulous fish retailer might switch the two.
Swordfish (Xiphias gladius) does not seem to have scales when one looks at a sample. Some say that it has scales that are embedded to such an extent that it is impossible to remove them without making a hole. Others say that it has kosher scales on parts of its body which fall off during its development. Still others claim that it may have some kosher scales even at the time of harvest.