As the title says, the Impaling enchantment for the Trident does not increase its damage against abyssal depths mobs. Mobs such as Zombies are considered Undead, which makes Smite deal extra damage to them. In the same vein, mobs such as Guardians and Salmon are considered Aquatic mobs. Impaling is supposed to deal extra damage against these creatures, yet none of the mobs from Abyssal Depths are considered Aquatic.

Fixing this would make the impaling enchantment a lot more useful with this mod installed (It's not very good in vanilla), and would make the trident an interesting way to combat mobs such as the Sea Serpent.

Sorry for not responding, I am using the latest 1.16.5-1.0.1

Which version are you using? These mobs should be classified as aquatic.

I have tested on the Swordfish, Sailfish, and Sea Serpent, however, I am not sure about the fish, since they don't have enough health to really be certain.

I know what the issue is. Swordfish, Sailfish, and Sea Serpents use separate entities for their body parts (which you can see by pressing F3 + B) that cannot be considered for enchantments. I think I can fix this by redirecting the player's attack to the main entity.