Pros:
-You have so much Burst on ADCs and mages early and late game.
-Good CDR
-One Shots
Cons:
-Youre Very Squishy
-Veigar destroys you
-Mana Problems
-If you miss your Q your whole combo is messed up

Flash: You should take flash (over ghost) beause a flash q combo on Nautilus stops your enemies from running away. Its also good for running off and flash juking.

Ignite: Take ignite to prevent your enemies from healing and slowly chip down their health for when they run away. Ignite is also very good for getting kills early game, and you need those kills.

Teleport: Teleport is also a very good choice because you can have a lot of presence on the map and gank a lot. Its very good for getting back to lane so you dont miss any CS.

Ferocity: None

Cunning:

Savagery: take Savagery because your passive works with your basic attacks so you deal more damage with your passive

Assassin: Because you are alone in lane assassin is a very good choice for taking down your enemy mid laner

Meditation: Meditation is a very good choice because you will run out of Mana very fast

Dangerous Game: You will be killing people a lot and running out of mana, so dangerous game is a great choice to keep staying in lane

Intelligence: You have very long cooldowns so having lower CDR will help you a lot

Thunderlords Decree: Take thunderlords for the insane burst. Your q,e,auto combo will destroy their health.

Resolve:

Unyielding: You are pretty squishy due to you not building armor so having bonus armor and MR makes this a great choice

Tough Skin: Because you are squishy this is also a great choice

Veterans Scars: The more health the tankier you are. This is a great one to pick

The more health regen you have the more you can stay in lane and get cs.

First and Second Back

On your first back you should be able to buy a Needlessly Large Rod and a Vision Ward. On your second back you should take an Aether Wisp and then another Vision Ward. Turn the Aether Wisp and Needlessly Large Rod into a Ludens Echo. You need the Ludens Echo first for the insane burst. If you dont have enogh gold and are forced to back before you can buy Needlessly Large Rod then you shold buy Aether Wisp first

Core Items

After Ludens Echo you should buy basic boots, and after that buy Sorcerers Boots. Then buy another Needlessly Large Rod. Then buy a blasting wand. Turn these items into Rabadons Deathcap. Now start building Zhonyas hourglass; buy Seekers Armguard first and then finish it into Zhonyas Hourglass (the activate effect on Zhonyas will be very helpful)

If youre Ahead

If youre ahead then buy a Sheen and then a Blasting Wand, then turn those into a Lich Bane.
After that you have the choice to build a Morellonomicon. First buy a Lost Chapter, then Morellonomicon.

If youre Behind

If youre behind then take an Abyssal Scepter against AP and a Liandrys Torment against AD. If youre taking Abyssal Scepter than build Negatron Cloak, turn that Into Abyssal Scepter. If you want to build Liandrys Torment then build Haunting Guise first. Take Liandrys afterwards.