AAUS is the American Academy of Underwater Sciences, and is pretty much the go to organization for scientific diving in the US. It was founded in 1977 in order to define scientific diving from commercial diving, and allow for an exemption in OSHA's commercial diving regulations. One of the many reasons for this was because OSHA had certain requirements for commercial diving, such as having a decompression chamber on site. You simply could not fund that with a science budget.
You can find out more about AAUS through their website.
About AAUS
AAUS works differently from your standard recreational agencies. It allows each establishment to operate as a sovereign entity, making their own protocols and procedures. This allows each establishment to customize their diving operations for their usual tasks and environments.
Because of this however, an AAUS diver from UC Santa Cruz can't simply dive for the Monterey Bay Aquarium (45mins away). They would need an orientation dive and/or a Letter of Reciprocity before they would be cleared to dive for another program.
Additionally I consider being an AAUS Sci Diver a rating not a certification, as you can usually lose your standing if you don't log a certain number of dives annually. There may be additional requirements as well such as CPR certs, First aid certs, and annual equipment inspections done by your program's Tech.
For each program there will probably be one rating: AAUS Sci Diver for *establishment*. You may get a NAUI Master Diver card, a different agency card, or you may get no card at all. I didn't get any card from AAUS, just one from NAUI with some special added text saying I was a "UCSC Sci Diver".
There may also probably be depth ratings for each diver. UCSC sets their ratings at 30ft, 45ft, 70ft, 100ft, and 130ft. You cannot make a scientific dive below your depth rating.
You can usually find an AAUS program at aquariums, universities, science museums; pretty much anyplace that would require scuba diving for scientific purposes. (ie. geology, biology, ecology).
Off the top of my head I'm sure the Georgia Aquarium has such a program. UC Santa Cruz has one as well as University of Oregon and Monterey Bay Aquarium. The best way to find one near to you is to checkout any marine laboratories/universities or major aquariums.