I actually main Nautilus,i play support mostly where he is very veeery strong,his kit is very simple,he has a lot of CC (E slow,Passive Root,Q Stun,R KnockUp) he is a very good tank with his W+the CC,
As jungler he is ok with his sustain but he has a very slow clear,his ganks are very good and once he hits level 6 is almost impossible to gank someone and not get a kill or an assist
As a top laner now,my diamond friend started playing him top last month and from plat 5 he went up to plat 1,he told me that he is one of the best tanks,but since we are in a meta where darius and other juggernauts are played it was pretty hard to win his lane,but he didnt lose either,it was even 80% of the time,also late game he was very strong because of his tank stats
His passive helps him cs underturret +the free cc you get (+some extra dmg)
His Q is a very fast almost non dodgable "skillshot"
His W makes him pretty tanky early,and his basic attacks deal some extra dmg too!
His E Has a very low cd and also slows down people
His R knocks people up in the air,also its an aoe knock up,if you R someone and he gets close to their team mates they will get also knocked up
Thats all i can give you,i main him support,i pick thunder lords with ap runes,i pick ignite or exhaust with flash or ignite-exhaust When i hit level 3 i go in with Q i auto them so i cc them in place (remember the extra dmg from my passive) i then use W (Which is an auto reset btw) (also since i run ThunderLords it will have proc by this time) then as they are about to leave i E them so i can slow them down,with this combo im able to deal the 2,5/4 of their hp,if my adc wasnt sleeping the whole time i just ignite them and like 95% of the time they will heal (also waste of spells because they are probably dead).Since ignite reduces healling effects they will get healled for what? 40 hp ? XD and thats all, most of the time its a sure kill,if not,sure summoner spells.If you hit level 6 do the same combo but instead of E use your ult then E
[Edit] And ty for the info about Rek'Sai