Drowned are not affected by the Impaling Enchantment

Drowned do not seem to take any extra damage from an Impaling trident, despite the fact that they are underwater mobs.

To test this, get a trident with Impaling V. If you throw at or hit a Guardian with the trident, it will die in 2 hits, whereas with an unenchanted Trident it dies in 3 hits.

However, if you try the same method on a Drowned, it dies in 3 hits from an unenchanted trident... and still 3 hits from the Impaling V trident.