Re: Is it Nautiluses or Nautili? - When referring to the Disney Nautilus - Neither.
The Vulcania Volunteers have also discussed this question at length & the plural of Nautilus pertaining to the species is Nautili, but in the case of a Proper Name of the Nautilus in a group or collection - the plural would simply be Nautilus. And in my opinion there's only One Nautilus!
Re: the efx display replica - I personally don't think the Best Nautilus model or replica has been made yet, but the 50th is by far one of better ones & Bob of Nautilus Drydocks is one the best Nautilus 'builders' that I've seen to date, ...above & below the water
As far as the grammatical correctness of the word, either is correct. While the original word Nautilus comes from the Greek word 'nautilos', the correct plural would be Nautiloi due to common rules of pluralizing Greek. However, in the mid-18th century when a grammatical movement to make english less irregular, and more like latin, words derived from Greek were given the standard '-us' ending thus making the plural Nautilus into 'Nautili'. However, once a word enters english it becomes an english word, thus having an english plural added and we get 'Nautiluses'.
So Nautiloi, Nautili, or Nautiluses are all, in fact, correct and so depends on personal preference.
Aside from that, that is an AWESOME collection of seafaring vessels.
Aside from that - Greek, grammatical correctness of the 'word' & personal preference aside:
whenever referring to the creature / cephalopod family Nautilidae, the previous reply is most definitely correct,however, ...when referring to the name of the submarine boat - Nautilus - & when refering to more than one Nautilus replica - Nautiloi, Nautili, or Nautiluses are in fact incorrect.
Whenever a title or proper name is changed it is merely being misspelled.