Mouth to snorkel works great if you just a simple J type snorkel (and dive with one). In fact that's the fasted way I know to breathe and transport a victim.
The problem with that is that a lot of the snorkel space in that scenario is dead air, which isn't really going to move that much, so in effect as you keep ventilating the patient with the snorkel, you're basically just blowing more CO2 into the patient's lungs and decreasing the amount of O2 that the body's getting. Now, it
may be enough O2 to keep the patient alive, but I'd have to see it to believe it. Although the patient is dead without any help so I guess it's better than nothing.
And yes, you can hold a good seal with a pocket mask with one hand. We're trained to do it routinely when giving CPR because one hand has to be on the mask while the other hand is on the bag connected to the mask. That being said, that's with the patient on the ground so obviously you have resistance when you push the mask against the patient because the patient can't sink into the ground, so that's an entirely different scenario than the patient floating on water where you don't have that same resistance. I would be /very/ surprised if someone could get a good seal without using both hands, and even with both hands, it's a toss up.
Unless you get the patient back within several minutes though, there's probably going to be some degree of brain damage, so honestly if you're boat diving the best thing is to get them back on the boat
as soon as possible, this means temporarily saying screw the CPR and hauling them up onto deck where definitive CPR can be given, and hopefully if one's on board, an AED can be administered which is what's going to bring them back; the CPR just keeps the body alive but without that electric shock to kickstart the heart, patient's screwed regardless. If you're shore diving...well, I don't want to sound pessimistic but unless you can get onto shore within a minute or two, that patient is probably not coming back anyway. The effectiveness of the AED decreases substantially with each minute after cardiac arrest that it's not given and there's plenty of studies done to prove this fact.
Moral of the story is keep breathing.
Forgot to add one tiny thing...look up the statistics on patients who are brought back from traumatic cardiac arrest. Medical cardiac arrest has crappy odds but it's still possible. Traumatic arrest has a 1% success rate because the CPR or the AED shock doesn't fix the reason the heart stopped in the first place. It's great to give CPR and try to keep the person alive or save them, but just be realistic, so if you don't get them back you don't get the notion that somehow you could have done more and it's your fault since it's not if you at least tried.
