In Bedrock Edition, when a drowned near a zombie nautilus, it will automatic ride on top, but in java, they does not.
MC-303842
Drowned cannot mount zombie nautiluses, unlike in Bedrock Edition
Open
1
(Unassigned)
1515231
Confirmed
Expansion A
Normal
Data Packs, Parity
25w44a, 1.21.11 Pre-Release 3
Retrieved
This is a feature request, so this is not the right channel, however you can report this suggestion on the official Minecraft Feedback website (https://feedback.minecraft.net/hc/en-us)