We were on the boat there for 5 days, snorkeled on 4 of them. I don't know if scuba would have added much more, we saw amazing stuff as it was, and I'm not certified to scuba (not actually sure if you need to be?).
We swam right next to a variety of crazy looking fish, giant sea turtles, rays, sea lions, and sharks. It was the highlight of my whole time away and if you ever get the chance to visit the Galapagos you must take it.
Sounds fantastic If you managed to see that lot whilst snorkelling. I can't say if you would have seen more with scuba but it seems you ticked lots of boxes.
One day I will go, definitely. I've heard it's crazily expensive to do a liveaboard cruise and there is also a waiting list.
It's all a dream at the moment though