According to MC-128249, drowned are not considered aquatic mobs due to them being undead. However, the zombie nautilus is considered both undead and aquatic and is therefore affected by both smite and impaling.
MC-305593
Zombie nautiluses are affected by Impaling while drowned aren't
Open
2
(Unassigned)
1543988
Confirmed
Expansion A
(Unassigned)
1.21.11, 26.1 Snapshot 2
Retrieved
This is because regulaar nautiluses are aquatic, but zombie are not.