This is really interesting - I've just gotten my packet to fill out to volunteer dive for the Florida Aquarium. I guess requirements vary. They require at least Rescue cert, and current CPR/First Aid. After they review the packet, if they would like to consider you, you have to undergo a AAUS (American Academy of Underwater Sciences) medical exam (!), take an in-pool skills test and written test. PLUS, attend several orientation / training sessions....
They don't have orientation sessions till January, though - but it'll take that long to get the rest of the requirements out of the way. I have the Rescue cert, but I have to renew my CPR/First Aid. (class is the 23rd) It appears that it's possible to get certified as a scientific diver through them.... and I think that just might be a good thing to do.
Wish me luck!