I think it's, frankly, ridiculous to think you would don flippers and snorkel gear while tripping so hard you can't remember to breath, then swim out into the water -- all the while breathing properly using the snorkel -- and then suck in water when you happen to dive such that you drown. Rather, that is, than go out with a crew of other scuba divers because you planned ahead not knowing better, as is most often the case when you scuba dive (being with many others, that is), and continue with your dive against your better judgment as a matter of conforming to the social norms of the other fully capable sober divers you don't want to embarrass yourself in front of and the urge to make good on the hundred or so dollars you've invested in the outing. I've never been pressured, by my monetary and social investment in the activity, to snorkel, because, well, it's snorkeling. It's pretty informal.
And as far as the cognitive demands of snorkeling versus scuba diving... Have you tried both? Scuba diving is far more demanding simply because of the probable demands of the situation (you don't typically scuba dive near the beach like you do snorkeling). I can't imagine anyone would even go in deep water snorkeling while tripping, because what would you or anyone you were with look at?... the vague dark blueness of the impenetrable depths you can't hope to see? I mean, we're talking about deliberately choosing to trip and go snorkeling, not being suicidally retarded to the point of cartoonish parody, right? When snorkeling the bottom would need to be visible and accessible -- and thus serve as a platform to push off of should you sink down to it -- or why else would you bother? I've been drinking tonight, but god damn it I'm pretty sure about this.