Hello people!
This is my first ever build for any champion, so try to go easy please. I have seen many other guides and what not for nautilus, and what he should be. Most have him as a AP tank, or jungler, or god forbids solo mid :/. Not saying that is bad, this is only my opinion, but nautilus has a great build as a HP tank. So, this is my idea as nautilus as a tank, please enjoy and leave comments below. Another note, I have use this build myself, so this is not just a bunch of ********. If you want to play with me aswell, type in Sesicman, and hook me up for a some games and tell me your ideas for this build.
NOTE: This build will not get you that many kills.

More or less straight forward. I build it off for health and health regain. Can't really think of any other explaination

Mostly health and defense. Now there is one point in the turret one. The reason becuase is that if you are tanking a turret with others, this will help the turret go down faster and safer for your teammates.

The more intresting part of this guide indeed!
For some reason, this system is not taking in Force of Natures effects! Your movement speed is acutally 407, and health reagain at 172. something per 5 secs.

Let start of with the first item, Locket of the Iron Solari. This item is great for many different reasons! The first is its area affect with your champ friends, health regain. Since the tank is going to be with the group most of the big battles, health regain is going to be important. The second reason is for the bubble of gold! Yes, if you press the 1 button, it puts up a sheild which abosrbes up to 50+10(per level) damage, so 230 damage at level 18. That is alot considering that helps that one greedy person on your team get away with 5 health left. Does NOT last for more than 5 secounds, so use wisely, and the person has to in range of your aura aswell. It also gives your champ health and health regain.
The next item is Aegis of the Legion. Most tanks get this item for its health and for the aura on the other champs. Its great, thats it!
The next item is Warmong's Armor. Do i really need to go into detail about this one? Don't make me hurt you... JK, its great for health and health regain
The next item is Force of Nature. This item is just great, Gives you armor, movement speed, and health regain. The price for it is also not bad either. There is no aura, but oh well...
The next item is Thornmail. Adds 100 armor and than deals 30 percent back as magic damage. So as the other teams is trying to kill you, you can know that they are hurting themself aswell.
The last item is the Mercury's Treads. The passive redueces all outside effects by 35 percent, and as a tank, your going to hit alot by snairs and frost and what not, so this helps you get towards or away from the fight better.

Q: Dredge Line
I take Dredge Line first becuase it helps get those who seem out of reach back to you and your teammate. I upgrade it towards the end since it does more or less the same damage, and it helps for those people who starts getting greedy into your turret or friendly champ.
W: Titan's Wrath
I take Titan's Wrath second for its sheild and area effect. I try to lvl it up as soon as possible, manly for its sheilding, but also so for area damage and basic damage. It deals up to 150 per second for 5 seconds, which can be more if you add AP, but thats not my guide :). This skill should be used for any situation, needed or not needed.
E: Riptide
I take Riptide third and level it up after titans wrath. This ablity is going to be your main attack and is going to be your disrupter in fights. Does three bursts of damage, slows enemys down, and is just awsome to watch as they are slowed down.
R: Depth Charge
Nautilus ultimate is a crafty one to say. IT ONLY TARGETS CHAMPIONS, so be stupid when thinking its going to help you with baron or any other weird thing. His ult travels to a champ, and send the person airborn and stuns for 2 seconds. If other minions, champs, or other things that move on the enemy team is in line with the ult, it sends them up, and slows them down for a short time.

Q your way in or out of battles. This stuns the champ or minions aswell, so it can help that one person get away without worries. Try foolowing up with W and E after. This slows the enemy team and they take damage at the sametime due to Titan's wrath. Use R for thos squishess in the back, to send all other champs in the air and acheive laughing, seeing all those peolpe in the air, cursing that one person in the back.
And repeat over and over until victory or defeat.

Read the How to use the skills before this please. The items are meant for tanking on the turrets and champs, while keeping your teammates alive while you take the hits. The items help help you keep you alive while your teammates deal the damage. Thornmail and Titans wrath deal a good amount of damage while you take the damage, so it is a win win for you. The bubble can help for those who want to turret dive or run away.

Heal because your going to take a lot of damage, and it also helps out your team, so be nice and help out that person trying to get away :)
Now surge is a funny one. Its mainly to get the attention of the other team and the turrets. It Speeds up you champ and adds bonus damage. To be honest, its really cool to see Nautilus really big and with Titans wrath on. He is just creepy looking.

Pros:
Very good for turret take downs and champions getting away( your side, not theirs)
Good amount of health regain and health
Great distraction
Good support champ
Fun to play as

Cons:
Slow when building, cost alot to get him to the top
Not a big hitter, so not really good solo
Not good against range kinda
Run out of mana fast if used unwisely