Thanks! Figured out where I went wrong in the key -- didn't notice that it said "lateral margins of elytron and pronotum serrate *or emarginate*" at one step...

Here's an interesting article (requires journal access, though):

Spangler PJ. 1961. Notes on the Biology and Distribution of Sperchopsis tessellatus (Ziegler) (Coleoptera: Hydrophilidae). The Coleopterists Bulletin 15:105-112.
http://www.jstor.org/stable/3998989

This particular beetle was collected from an upstream wooded section of an otherwise fairly impaired urban creek. The site was consistent with published descriptions of its usual habitat: sandy soil, undercut banks, woody debris in the stream channel.