Alban sent over a link to a very nice set of plans for an S-class submarine posted on Flickr by "daviddb". I'm pretty sure this is the S-class produced by Great Britain, which was a different design from the United States S-class. It isn't an accurate historical resource for the Innsmouth submarine, but the full-sized scan at the link would make an amazing prop or handout.
Be sure to browse the rest of the gallery as well. There's some incredibly cool stuff there.