Steampunk was more or less invented by Harper Goff, lead designer on the 1954 Disney 20,000 Leagues Under the Sea. Goff is quoted as saying "Nothing looks more attractive than a combination of rough iron, and elegant luxury." Which if that doesn't describe steampunk, I dunno what does.
Also, while the original design already looks quite steampunk:
It turns out Goff's original study model looks even more steampunk:
The Nautilus is technically nuclear powered (in the movie not the book), but based on the machinery we see in its inner workings its using the nuclear reactor to drive a steam turbine rather than charge batteries.
The term obviously came about later, but the 1954 Nautilus has all the stuff we normally assocaite with modern steampunk like big pistons, rivets, oxidized iron or copper plating, port holes, an exotic power source of some kind driving a steam engine, and of course a tasteful 19th century Victorian-style interior. All that's really missing is the giant exposed gears that ended up being the death of steampunk anyway.