The bug
A drowned hit with a trident deals 9 damage to it. It deals the same damage of 9 when the trident is additionally enchanted with "Impaling V".
Linked issues
is duplicated by 5
MC-129940
Drowned are not affected by the Impaling Enchantment
Resolved
MC-132515
Drowned Are Not Affected By Impaling
Resolved
MC-188011
Impaling is not affecting trident damage
Resolved
MC-238753
Drowned are not water mobs
Resolved
MC-273747
A Trident with Impaling doesn't deal extra damage to drowneds
Resolved
Attachments
Comments 5
Dolphins now get affected by Impaling but this Bug was never marked as "Fixed"!
Drowned are not affected by Impaling in 18w20c.
Whoops, marked the fix version incorrectly, thanks!
It works as intended because Drowned, while happy underwater, are classified as undead mobs rather than water mobs and therefore aren't affected by the impaling enchantment.
Impaling also doesn't help against Drowned, in either ranged or melee attacks. Tested with levels 4 and 5 in 18w19b (works fine against Guardians).
Presumably the same problem, didn't want to duplicate, but perhaps title and labels should be revised.