You didn't mention any specific destinations, but I'd like to recommend Mike Haber & Mike Mesgleski of Jim Church School of Underwater Photography. http://www.jimchurchphoto.com/index.html
Most of their workshops are aboard the Aggressor fleet, which as you most likely know, is scattered all over.
Their style of teaching is very effective, and there's a good mix of lectures, one-on-one time, and assistance in the water. It's a worthwhile investment, and you'll spend time with some fun folks.