Hi Evan,
I can commend on the workshops of Alex Mustard, I have been on two of them. He is not just an outstanding UW photographer, but also a focused and patient teacher (no matter at what level he is teaching), who has the gift to explain complicated things in a simple way...
It is right that top photographers (even winners of great competitions) participated, but most participants were regular UW photographers. The experience was extremely positive, I (and also my wife) learned a lot and certainly will join Alex in the future again. In order to get a place in his workshops, you better be on Alex's mailing list (usually the workshops are booked out within few hours after announcement) and this concentrated expertise comes at a price.
Maybe it is more economical to first join a (more budget friendly) workshop at Backscatter/Bluewater and then attend one of Alex's workshops, in case one has zero UW photography experience. Lisi and me studied the books of Alex Mustard and Martin Edge, practiced approx. 2 years and then attended our first workshop...
(It is alike with the diving courses: when you make AOWD just after OWD the progress is small, but when you practice a lot after the first level entry course, you are really prepared for make maximum profit from the next level course)