I'll chime in with my perspective at the moment
BS in Agricultural Engineering and also a BS in Civil/Environmental engineering. Did some time as a geotechnical engineer (8 months after graduation, 4 summers prior as a tech). I've been with state regluatory since working wtih wastewater, storm water, and air quality...mainly NDPES permits, and water quality is my strong point.
I took the Civil PE Exam with Water Resources emphasis twice so far, and have failed both times. My diagnostic reports (or whatever!) say I need to study transportation and geotech for general sections...which I hit hard-core for the second exam in October '13, so I felt very discouraged. I aced water quality the first go-around in April '13, but there weren't any questions at all in October...so sometimes you just luck out I think on what the exam focuses on
I decided to start with Civil PE exam, since my thought was I'd just have to "relearn" or study the general topics, whereas with the Enviro exam, I'd have to learn "from scratch" solid and hazardous waste, and basically air quality, too, since I'm not very confident I know much of that . I had a friend who had all the civil study material for me to borrow, too, so that helped Plus, her emphasis was transportation in school, but all her real world experiance was structures...she took the transportation afternoon session and passed.
Anyway, 3 coworkers took the Enviro exam in October '13...two were NPDES-background (chemical and mechanical degrees), one was hazardous waste background in the department. So they all had to learn something new, and they all passed first try =) So there is hope!
So I figured since I was guaranteed some water quality questions on the Enviro exam, I might as well give that a go, since for me, Civil Water Resources has not been working (darn transportaion!!). Fingers crossed third time's a charm and I'll pass the Enviro exam in April '14!
Bottom line, you get to put fancy letters in your title and stamp things (if needed) regardless of what your PE is in...at least that's my thoughts, but that's also coming from employ with state govenment.
On a side note, my friend who passed the Civil PE with emphasis in transportation uses her license to stamp/approve structural designs...