How to tame a nautilus and get nautilus armor in Minecraft
Netherite Nautilus Armor

Netherite Nautilus Armor: 19 armor points, toughness 3, immune to fire; made from the diamond one and a Netherite Ingot. Nautilus armor protects a tamed Nautilus, the mount you ride underwater. All nautilus armor works the same; the Iron Nautilus Armor page explains it.
It is made with the Smithing Table from Netherite Upgrade, Diamond Nautilus Armor and Netherite Ingot.
Netherite Upgrade + Diamond Nautilus Armor + Netherite Ingot → Netherite Nautilus Armor
// Slots with several items cycle through every accepted option.
Netherite Nautilus Armor
At the Smithing Table: 1 × Netherite Upgrade, 1 × Diamond Nautilus Armor and 1 × Netherite Ingot give 1 × Netherite Nautilus Armor.
It protects a ridden nautilus in underwater fights. Worn by: Nautilus and Zombie Nautilus.
No: only one per inventory slot.
No: as a dropped item it resists fire and lava.