How to tame a nautilus and get nautilus armor in Minecraft
Then put a Saddle on it and ride it, and you stop losing air underwater.

The Saddle lets you ride horses, donkeys, mules, camels, pigs, striders and nautiluses. It is crafted from Leather and an Iron Ingot and fits in a special slot of the mount.
It is made with the Crafting Table from Iron Ingot and 3 × Leather.
Use it on a tamed horse, donkey or mule, or on a camel, to ride and steer it. A pig needs a Carrot on a Stick to be steered, and a Strider a Warped Fungus on a Stick, which lets you cross lava lakes in the Nether.
Shears remove a Saddle from a mount, and a saddled animal drops it when it dies.
Saddles also turn up in chests of Nether fortresses, villages and End cities, Ravagers drop one and you can fish one up; Leatherworker villagers sell them too.
3 × Leather + Iron Ingot → Saddle
// Slots with several items cycle through every accepted option.
Then put a Saddle on it and ride it, and you stop losing air underwater.
Without a Saddle you sit on the horse but cannot steer it.
dyed leather armor, Saddles
every Evoker drops a Totem of Undying, every Ravager a Saddle.
At the Crafting Table: 1 × Iron Ingot and 3 × Leather give 1 × Saddle. It can be found as loot in Nether Fortress, Bastion Remnant, Abandoned Camp, Village and 1 more. It is dropped by Ravager. Sold by Villager: Leatherworker (Master). Fishing: Fishing Rod.
It lets you ride and steer horses, donkeys, mules, camels, pigs, striders and nautiluses. Worn by: Camel, Camel Husk, Donkey, Horse, Mule and 6 more.
No: only one per inventory slot.