Anyone interested in doing more on a dive vacation than just looking at pretty fish and marine life should check out the Marine Specimen Collecting Expeditions that the New England Aquarium in Boston undertakes usually twice a year and usually to the Bahamas. I have personally been on seven of these trips and thoroughly enjoy them. Very different - educational, fun, exhausting - all my criteria are met!
I just finished producing a promotional video for the aquarium on these trips. You can view the 10 minute video at: http://revver.com/watch/1160786