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.