Undine and Rusalka are exceptions that throw a wrench into whatever categorization logic I can come up with. Those two really should have been eidolons. Apsaras too I guess, since Apsaras is more of a category, like nymphs, than an individual divinity. Also, they're usually wives of the Gandharva, an SR eidolon.
Takemikazuchi's a deity; should be a kamihime instead of an eidolon.

So it's easy to come up with general rules of thumb, but there's always a few nagging exceptions.