First of all I would like to mention that this is my first guide (I should also say that although I just joined I've lurked for quite a while 0_0). This stated, I would definately apprciate constructive feedback. Nautilus is a pretty versatile tank, I've had good success with him solotop and jungling but nothing is more fun that the hard CC and the ability to soak up tons of damage on bottom lane, so that's what this guide will focus on. I may add more builds for different situations later.

Pros:

Very tanky with his Titan's Wrath
He is a CC master with his Dredge Line, Riptide, and Depth Charge controlling the battle
His Depth Charge, Dredge Line combo provides great chase capability

Cons:

He's pretty slow without using his Dredge Line to move around
His Dredge Line can sometimes be tough to land
Teamwork is critical for him to be of any use

Let's jump right in: why these runes? They provide the best synergy with his abilities and the support role we are trying to have him fill, although there are some other options as well.

Greater Mark of Health: This is great because it synergizes with his Titan's Wrath which scales the armor buff off bonus health, besides the fact that more HP is always good on a tank.
Greater Seal of Scaling Ability Power: Adds some late game damage to all of his abilities which don't scale to bad for a tank. (This is the only option I feel is viably substituted, another option might be Greater Seal of Armor)
Greater Glyph of Cooldown Reduction: Very important to keep up the CC and harrass on your lane opponents early game, and equally important to keep your armor buff on Titan's Wrath which has a VERY HIGH cooldown at lower levels.
Greater Quintessence of Gold: Some consider this a staple for all 0 CS supports, but it is made more important on Nautilus as this build doesn't get any GP5 from masteries.

I go with the 1/21/8 setup since as a support/tank Nautilus will be harrassing and soaking lots of damage and will need the extra defense. You can play around with where you want to put the points based on your play style but the extra HP from Veteran's Scars and Durablilty are strongly recommended again because of the scaling on Titan's Wrath. Also you may want to change where you put the points for Summoner Spell buffs depending on which ones you use.

I usually start with Regrowth Pendant + 1x Health Potion since again as support tank you will need that early sustain.
After you have enough cash go back for Philosopher's Stone, Vision Wards and, if you can afford it: Boots.
This I usually follow with a Heart of Gold.
After I've upgraded to Mercury Treads, I go for a Rylai's Crystal Scepter starting with Giant's Belt again for the extra HP and scaling on Titan's Wrath.
Next I'll usually go for a Glacial Shroud for the Armor and CDR, and then Abyssal Mask for the AP and MR (and equally important since you are also support, the MR debuff).
At this point it's probably a good idea to finish off the Frozen Heart and depending on how much gold you have you can make Locket of the Iron Solari and Shurelya's Battlesong or go for something else with like Sunfire Aegis, Zhonya's Hourglass, or Guardian Angel.

A comment was posted asking why I chose Rylai's Crystal Scepter and Abyssal Mask before Locket of the Iron Solari or Shurelya's Battlesong. You very well could choose one of those first but this is why I ordered the build this way:

The reason I went Rylai's Crystal Scepter first is mostly for the Giants Belt and additional CC. It adds a bit more sustain which you will need for mid game teamfights. the Abyssal Mask could wait if you don't need the extra MR, but another reason is that I'm a little apprehensive about letting go of your gp5 items too fast. if you are getting enough gold from assists, then yes I would go for Shurelya's Battlesong or Locket of the Iron Solari first, but chances are your going to need to be a bit more tanky, so I would at least get the Giant's belt and then one of these before moving on.

It is important to get Titan's Wrath first since your other abilities require you get in close to use them. It will also be a major damage dealer for you with each hit dealing DoT. At level two it is a good idea to get Riptide as it can get your carry out of some sticky situations with it's slow as well as provide some low range poke. At level three Dredge Line is a good idea in order to set up for ganks and catch a lane opponent out of position.
After this though, max Titan's Wrath first, the lowered cooldown per level is very important for sustain in the lane.
This should be followed by Riptide for the added slow per level.
Dredge Line should be leveled last because until you get some real AP it is just for the most part a closing utility.

Summoner Spells are largely dependent on your playstyle. I chose Heal obviously since support should have some way of helping to sustain their carry. My other choice, Exhaust is also very good since it can either add to your CC arsenal in locking down an opponent for your carry to finish off or allow you and your carry to escape a gank by neutralizing a major damage dealer. Other viable options would be Clairvoyance, Teleport, or Cleanse if you find yourself against a CC heavy team.

Again, these are not the only options but I feel they fit best with the role you are playing and the kit Nautilus has at his disposal.

As a support and CC heavy tank, Nautilus is all about teamwork. like Amumu, Rammus, and Shen, he can swing the tide of any team or lane-fight if he works together with his team or lanemate to faceroll the enemy. If he can't work with his team, or the can't work with him however, he becomes useless. He is a great initiator with his Depth Charge that can knockup multiple enemies which he can then follow with Dredge Line to either pull away the enemy carry or jump into the thick of things to deal out some AoE with Riptide and Titan's Wrath. These abilities will make him very hard to ignore, but they are unlikely to kill any enemy champs which is why the carries on your team need to be aware of when you engage and take advantage of that opening.

It also means that you should not engage the enemy without your team around. Yes you can take a lot of damage while you dish some out, but usually the trade will not be worthwhile in most situations.

Nautilus's kit allows him some interesing opportunities for poke and harrass which is rare in a mele champ. His passive Staggering Blow with it's stun allows you to pop in with Dredge Line, Titan's Wrath activated hit the enemy and Riptide while you bate them into chase with minimal to no damage.

It is also important to remember that 1 second stun in a teamfight. Since Nautilus should be right there in the thick of things anyway spamming Riptide and so on, try and hit as many enemies around you as you can proc'ing Staggering Blow.

Using Depth Charge on fleeing enemies is very usefull also, as it will follow them until it strikes, knocking up and damaging anyone that it hits. If you can target an enemy fleeing they will usually try to put their teammates between you and them allowing this easy pickup in extra cc and damage.

Hopefully you have found this guide to useful in both tactics and build strategy. If you have any comments or suggestions please feel free to let me know. Again this is my first guide on Mobafire and would like the opportunity to improve it if needed rather than getting down-voted into oblivion.

Thank you, and good luck!