Hello, It's ya boy strongest whiskers in the game here to fuqk your ADC up long-game style.
I'm a main support who has recently discovered the beauty of Nautilus. This is my first guide, I hope you like it and I cant wait to hear if this has helped you in anyway.

I'll continue to work on this.

Champs I feel go great with Nautilus, in no particular order

1. Xayah
Her kit is really good, not the greatest ult and might need to use a little time farming before she becomes a killing machine, her ability to root combinded with your passive is a deadly combination

2. Jihn
I love laning with him because of his ult, whether in your bot lane or in a taem fight, go all in and get them to attack you, use your E to slow them all, hook who ever is trying to excape or ult them yourself for a Penta.

3. Miss Fortune
You compliment her a lot in team fights, slowing everyone in her sight or hooking that annoying character as a set up for her ult.

4. Ashe
Her slows are sick your slows are sick, they won't be able to get away if she's by your side

5. Jinx
What can I say, she has her chompers, she can slow them with her Zap! you can compliment her ult with yours, match made in heaven.

Here I'll try to explain why I chose these Runes

Resolve

Aftershock:
Both your passive and your hook immobilizes the champion you attack, since you're a support you will benefit from the defenses bonus and the later burst dmg that will occur, making sure the enemy is as low as possible once your ADC sweeps in for the kill.
Font Of Life
Once again this rune plays off your ability to impair the enemy champion, your hook and passive basic attack does both, this works well for those pokes that arent all in and gives your ADC more time in the lane staying ahead with the heal they gain.
Conditioning
This just felt like the best out of the bunch, you're firstly a support but secondly a tank, you will need the bonus +8 armor/+8 magic resist
Overgrowth
This disgusting rune will make you a monster, just stay with your ADC while he/she farms and watch your heath grow larger and larger. Just always be around the destruction of your surroundings and soon you will be tanky without even buying tank items!

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Secondary - Inspiration

Let me start off by saying I've been playing a lot around with the secondary rune for this build and I havn't pinned down what is the overall best yet.
Future's Market
You are no longer restricted if you're up against a rough lane, being able to buy items and core items before your enemys is key if you want to win and eventually snow ball into victory. Just make sure to buy the items you need and don't waste being able to get those boots or whatever before your enemy lane.
Approach Velocity
This helps me a lot when Im entering a team fight and want to get close enough to use my support items, it also works really well if you're in lane or team fights with a lot of CC to make sure you can hunt down or hook who ever is trying to flee.

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Other runes that didnt make the list

Biscuit Delivery (Inspiration)
A really good rune if you want to make sure to stay in your lane as long as possible, Nautilus get very many deprived quickly and i'd pick this up instead if you're heading into a match that seems a little rough.
Just remember to eat those biscuits they will save your life.
Cosmic Insight (Inspiration)
I frequently switch between this rune and Approach Velocity, I really like to use my support items and want them to be up as much as possible when you're entering the mid - late game. With the extra CDR it can really be a game changer.
Iron Skin Resolve
You might want to consider swapping this bad boy instead of Conditioning, I find it very underrated and valuable since the early game is crucial

I'd like to explain why I choose to play Nautilus this way, first of all you are a support not the teams tank, this doesn't mean that you aren't supposed to absorb as much dmg from your adc as possible but to put you in the right mind-set of your role. This is why I rarely buy tanky items compared to other "tank supports". You want to stick by your ADCs side and make sure he gets the kills required to become strong enough later on.
With items like Zekes Convergence and Knights Vow you guarantee that. You want to keep him in the lane as much as possible and stacking up as many kills as possible to push the game in your favor.
I recommend buying Boots Of Mobilityif you'd like to roam and ensure victory in as many lanes as possible. The other boots are fine depending on what set up you're facing.