Impaling is an enchantment that is exclusive to the trident, this enchantment will increase the damage dealt to aquatic animals or any mobs that are in the water. It functions like how Sharpness works for a sword or ax. Its highest power level is V (5).
An Impaling enchanted book (via screenshot)
When dealing damage to aquatic mobs or any other mob that is in the water you will deal extra damage up to 12.5 at level V to them. In Java Edition, only aquatic mobs receive extra damage. Aquatic mobs include axolotls, dolphins, guardians, elder guardians, squid, glow squids, turtles, and all variants of fish. Drowned dont count as aquatic mobs so they dont receive extra damage from this enchantment.
A squid killed with a single hit from a trident enchanted with impaling (via screenshot)
The easiest way to obtain impaling is through the enchantment table, you will require Lapis Lazuli and high enough XP, and a trident to enchant. You can increase the chances of receiving a higher power level of Impaling by placing bookshelves around the enchantment table. Other ways include fishing, trading with villagers, looting chests, and using an anvil to apply an enchanted book with impaling on it.
An enchantment table offering Impaling I (via screenshot)
If you are planning on an aquatic adventure or just exploring the oceans this enchantment will come in handy. Its increased damage will help you defeat foes quickly while keeping them at a distance from you.
A. The highest power level of Impaling is level V (5).
A. No, It is an enchantment that is exclusive to the trident.
A. Impaling will do 2.5, 5, 7.5, 10, and 12.5 at levels I, II, III, IV, and V respectively.