I absolutely agree with Stephen Ash. Neither CPR nor rescue breathing is going to be very effective in the water. It MAY be worthwhile to try one or two breaths, to restart spontaneous respirations. But frankly, if you are without respiration or pulse in the water further than a few feet from a boat or land, you are not going to do well, and in my own case, I would rather hope nobody succeeded in resuscitating me under those circumstances.
Where the rescue training IS valuable, I think, is in learning to manage situations which have not deteriorated that far, and preventing them from getting there.
Diving rescue scenarios have far more in common with what paramedics do than what I do. What I do is in a very controlled environment, rich with resources and relatively rich with assistance. Paramedics have to be prepared to face all kinds of variables in environment and events, with minimal equipment and resources, and under intense time pressure. What the two sessions of Rescue we have done so far have shown me is how challenging assessing the situation, evaluating the condition of the "victim" and formulating the best strategy for dealing with the situation can be.
What I mentioned to my instructors and still feel is something not well covered in the class is the recognition of a diver who is LIKELY to get into trouble, before the trouble happens. The book talks about recognizing a nervous or stressed diver pre-dive, but I suspect there are a number of indicators that can be picked up underwater as well. I know from my own experience that rapid respiratory rate, labored or inefficient kicking, or persistent buoyancy problems can indicate heightened anxiety. In my case, failure to "scan" with my light often indicates I'm dealing with something untoward (eg. flooded mask, equipment malfunction). I'm not terribly good at picking those subtle kinds of things up in my buddy yet, although I'm working very hard at it, but people like Bob and my tech diving lady friend are eagle-eyed.
At any rate, although I may differ as to the advisability or utility of some of the interventions recommended, that's not a lot different from the ACLS or ATLS protocols

I still think it's well worth while to practice assistance scenarios -- for example, I discovered last night that, if I have to haul my husband to shore, it can be quite difficult to keep him centered over his back inflate BC, and if he begins to fall to one side, I don't have the body weight to swing him back very easily. Instead, I need to get under his shoulder and lift him that way. Good thing to know before I need to know it!