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.
Comments 3
Thank you for your report!
However, this issue is Working as Intended.
Please note, that mechanics of the game may change between updates.
Things such as graphics, sounds, world creation, biomes, redstone, villagers, and animals may not work the same in current versions.
Full Version History -- Snapshot Version History -- Feature Requests and Suggestions
Quick Links:
📓 Bug Tracker Guidelines -- 💬 Community Support -- 📧 Mojang Support (Technical Issues) -- 📧 Microsoft Support (Account Issues) -- 📓 Project Summary -- ✍️ Feedback and Suggestions -- 📖 Game Wiki
This is because regulaar nautiluses are aquatic, but zombie are not.