The Adeptus Nautilus started as an entry in an Instagram painting contest. The theme was underwater, and with not many varieties of figurines left in my backlog to sacrifice, I ended up using a space marine, because I still have tons of those. Space marines, despite their name, does not very fit in an underwater theme. So the very first idea I had that wouldn't ask for too much kitbash or sculpture was a diver ! An old school diver. Inspired by those old (and kind of creepy) diving suits with a hint of Jules Verne aesthetics. Of course, despite the name of the chapter not following the rules of any space marines chapter, I couldn't help to name him Adeptus Nautilus, because I'm a sucker for rhymes.
Now, I didn't plan for it to become a thing, falling for the poster boy isn't really my style, although it is true I do love to take the mainstream thing and personnalize it to death. After all, I did see my partner's space marine kill team and thought "I kinda want one of my own, I'd make them pirate hat". So one day, when I got kinda tired of painting something really precisely, I looked at my Adeptus Nautilus and thought maybe I should make him a little friend. And here I was, dugging up some old space marine and taking an old little scout to my desk. I felt brave enough to start sculpting with green stuff, and that was only the beginning.
I don't know much about 40k lore, that is true - well, I certainly wouldn't know enough to feel confident about writing a lot of context and little stories about my Adeptus Nautilus friends. So I'll just stay on a very general level.
The Adeptus Nautilus is a Space Marine chapter specialized in water planets. Whenever there are some Xenos problems on a water planet, it's whom you call. They have specialized armor and weapons. They have armor and vehicles that can let them go underwater, and some of them have a cool power trident.
Aesthetically speaking they have everything that reminds of sea.