Nautilus is one of the best engage CC tank in the game. Nautilus has so much CC in his kit and he is super simple to play as well. Nautilus has 4 different forms of CC in his kit.

His clear is now decent with pet buff and last season buff to his E.
You can full clear starting from blue buff in 3.21 and 3.23 from red buff (patch 14.3).
I path top to bot 99% of time.

Pros

- Lots of CC in the kit
- Very simple kit to learn
- Strong ganks
- Hard to avoid his engage
- Can peel a lot in teamfights
- High burst with low counterplay with all the CC when ahead

Cons

- Falls off late game but not that much with this build
- If you get behind, you won't be that tanky
- Big hitbox, making it easy to get hit by skillshots
- Low dps so still need to rely on dps allies to win
- Easy kit but hard to play in the jungle as you need to hit your Q for ganks (I don't advise him for low elo and beginners) and clear is not that easy when learning the champion

The point of this build is to actually have damage and not depend too much on your team to win games.

You will add damage to your full CC kit to target carries and then peel for yours.

This build is about HP scaling.

Aftershock, shieldbash, heartsteel, sunfire cape, radiant hollow, unending despair and titanic hydra all scale of HP. Abyssal mask too in a way.

The 14.1 changes to the jungle and especially the pet damage allow you to not buy your first item to increase your clear speed as it will deal huge damage just by itself, that's why we can rush Heartsteel and start to get stacks for it.

You will also have way more interactions in the jungle with all the objectives to get stacks.

I often build as following :
- T1 boots + dark seal on my first back
- rush Heartsteel or Liandries based on enemy team
If you think you need more damage or more tankyness - for info as a first item Heartsteel give aroung +25% ehp than Liandries when Liandries give +30% champion damage and clear speed.
You can also take into account that you can get around 100 and 200 hp from HS during the time you finish it and your next item.
- finish boots based on enemy team
- get Liandries or Heartsteel depending on first item
- get between half and 2 defensive items based on the enemy team and how fed I am
- build Titanic hydra
- complete with defensive items

Since you are a tank, you can change your core build if you really need more defense but that mean you are loosing hard which is not a good base for a guide.

Core build explanation :

Why do I pick Liandries Torment and Heartsteel ? (instead of bami's item and another tank item)

Let's have a look at Nautilus AP ratios :

Q : 0.9

W : 0.4

E : 0.5 (up to 1)

R (single target part) : 0.8

Since E has a really low cooldown, you can combo with Q, E mid air, auto (passive), auto (W), auto (still W), R, auto into E again.

That mean you will put between 0.9 + 0.75 + 0.4 + 0.4 + 0.8 + 0,4 + 0.75 = between 4.4 and 4.9 if you hit twice 3 waves E (unlikely but possible).
That mean 90 AP from liandries is 4.4 * 90 = 396 damage.

During that time, you will apply Liandries dot %hp damage for approximatively 10 seconds (7s combo + 3s damage over time).
So that's 0.02 * 10 = 0.2 => 20% max hp of your target max hp.

At level 9 when you get the item, most champion have at least 1300 hp.

So 20% of 1300 hp => 260 damage minimum aganist target with no hp items.

So that's 656 more damage. In comparaison, sunfire deals 12 + 0.0175% max hp / s.
The item itself deals 12 + 0.0175 * 450 = 20 damage / s and increase by 10% each second.
So it's (1 + 1.1 + 1.2 + 1.3 + 1.4 + 1.5 + 1.6) * 20 = 182 damage.

Also Liandries deals 50 damage / s to monsters because of the cap. And you have to add the damage you get from the AP too.
Sunfire deals 20 damage / s to monsters up to 32 against epic monsters.

For info, sunfire need 685 hp to double it's damage.

My conclusion is that sunfire is a bad first item for clear speed so I prefer to buy Liandrie for clear speed and damage against champions and buy a bami item later when I have enough HP. Especially considering because Nautilus has high AP ratios, bad clear without damage and low damage against champions without any damage item.

Why Heartsteel ?
You have a part of the answer above as I want to have hp for my bami's item, but you have to add that it has also a % items hp ratio (12% next patch) on the item and combined with Aftershock (8% max hp), Shieldbash (1.5% x2 proc with green pet and W).
This with the bami's item, will make you scale a good amount with HP.
I consider Heartsteel more like a damage item with a good amount of HP than a defensive item.

In general HP is considered like a bad stat for tanking but I never really feel like I'm not tanky enough with these 2 items. And you will also have green pet, and all the resists from the runes.
Not optimizing for being as tanky as possible is not the same as not being taking at all.
You will be a bit less tanky than a full tank build but will deal around 50-60% more damage on a full combo.

You will get resists on your 3rd and 4th item. I highly suggest you buy a bami's item if you don't need pure defensive items. I really like the Hollow Radiance + Frozen Heart combo.

Aftershock is very good with this build since it gives some resistance that will scale well with all the Hp from the build and deals damage based on your bonus HP.

You should take shieldbash as it works with green pet to give you a second proc of shieldbash.

Conditioning gives "free resistances".

For the last Resolve rune, you can pick the 3 different runes based on your team and the enemy team.

Overgrowth is the standard rune because more tankyness.

Unflinching is good if the enemy has a lot of CC.

For secondary runes, you can take domination page with eyeball collection or cheap shot, treasure or relentless hunter.
You can also take precision with triumph and tenacity.
I generally prefer to take inspiration with hexflash and future market.

I use treasure hunter to be able to rush Heartsteel as fast as possible.

Passive : not much to say, switch target to apply it as much as possible to all the enemies. Try to chain CC and not apply all your CC at the same time.
You can use it to apply the 3 waves of your E (see below).

Q :
The cd is reduced when you hit walls, use it constantly in the jungle as you will have infinite mana for it. It's especially good for side lanes if it's pushed as you can Q, auto for the CC then E and you will have it soon after they flashed.
It can counter displacements like Blitzcrank, pyke Q, gragas R etc : if you Q a wall or an enemy just before the enemy displacement affects you, you will be pulled to it.
You can also use Q to force E 3 waves.

W :
It's an auto attack reset so use it just after your first auto attack.
The damage from it will last as long as you have a shield even if it's not from the skill. That's why green pet is good.

E :
You should try to hit 3 waves as often as possible especially early game and in the jungle since the damage will fall off later.

You can force 3 waves with one max range auto with passive then by going away from your target just 1/8s and pressing E.

You can also use a max range Q and press E the exact moment the anchor is touching your target. It's hard and require practice.

R :
There are a lot of different ways to use it depending on the situation.
Sometimes it's best to engage with it as you know your target won't be able to escape.
But most of the times if you engage with it, your target will just run away or flash or use a dash and it won't be really effective.
You should try to hit your Q and R later.
During a teamfight, you should always try to use R to a target behind the first enemy you hit with a Q(+E) + passive.

Combos :
The simplest combo is :
Q, auto(passive), W, E, auto

With ultimate you can do the same but with R at the end :
Q, auto(passive), W, E, auto, R, auto

With ultimate with less uptime CC but no counterplay (you don't want the enemy to flash or dash) :
Q, auto(passive), R, auto, W, E, auto

With ultimate but you optimise your E cooldown for more damage (need some ability haste, at least 30) :
Q, E during Q, auto(passive), W, auto, R, E, auto

With Heartsteel and Titanic Hydra :
Q, auto(passive), W, E, Titanic active (+Heartsteel)

With ultimate, Heartsteel and Titanic Hydra :
Q, E during Q, auto(passive), W, Titanic active, R, auto(heartsteel), E, auto

His clear is now decent with pet buff and last season buff to his E.

You can full clear starting from blue buff in 3.21 and 3.23 from red buff (patch 14.3).
Here is the link with videos (patch 14.3):
https://www.reddit.com/r/NautilusMains/comments/1aptut2/nautilus_full_clear_videos/

I path top to bot 99% of time.

In the jungle, you get most of your damage outside of the jungle pet from your E.

This skill can actually hit 3 waves each dealing damage (2nd and 3rd dealing 50% damage).
Hiting 3 waves will allow max damage so it is very important to know how to do it and sadly it's not really easy.
You can learn how to do it in this video at 1.55 : https://www.youtube.com/watch?v=nBCPHQ6NWeI.
His E max damage against monsters is 165 / 270 / 375 / 480 / 585 (+ 150% AP).

With his W shield, he is really healthy during his first clear.

At 3 camps, you have the choice to continue farming with 2 points in E or to gank with Q.

With the recent changes to the jungle and the buff to his E, his clear is good enough to take 6 camps before scuttle spawn.

The most important to improve your clear speed as Nautilus is to learn how to double camp and not waste your E only for 1 camp if you can apply the damage (and the damage over time from the pet) to 2 camps at once.

Of course Nautilus is an awesome ganker so you should gank instead of farming but only if there is a good opportunity for it.

I prefer to clear top to bot because as a tank, the squishy bot lane is far easier to take down whereas some top laner can really snowball to the point you can't gank them.
Toplane is harder to gank in s14 also.

You also prefer to play around voidgrubs and rift herald instead of the dragon.
In general you can skip the first 2 dragons if you get voidgrubs and herald and then start to play around them later.

During the early game, you want to gank lanes that are pushed because Nautilus is not really good for dives unless the enemy is low.

You want to run to the enemy laner to auto and root them instead of engaging with your hook as much as possible.
You want to wait for the flash before to press Q.

If possible, I want to run at the target, press E then auto => W for the auto attack reset and wait for the flash.

If you have to engage with Dredge Line then do it, it's just not the best scenario.

At level 6, it is the same. Only engage with your ultimate if you are sure that if will be a kill, otherwise try to run at the target E, auto, W and wait for the flash then Q then your passive might be back again then press R if needed only.
It's really a devastating gank almost impossible to avoid for a lot of champions.

You should also use your hook on the walls if the enemy is close to a wall. It will reduce your Q cd by half and even at level 1 you will have it available to hit a champion during the gank. It's especially good on side lanes as the walls are close to the brushes and your enemies will most likely run through it.

I will continue to improve the guide over time but don't hesitate to leave a comment to give your feedback if you tried it.