To pull the analogy card, I have been a "sound guy" for many years. Church services, weddings, funerals, class plays, coffee shop sessions... I've seen more than my fair share. Ask any sound guy, and they'll tell you that running sound is hours of tedium punctuated by moments of sheer adrenaline (which is what pilots and all sorts of other people say about whatever *they* do, too).
Anyway, I had several sound guys working under me at various times and places, and most of the time, you'd be hard pressed to tell the difference between us. Most of the time, running sound is just getting what you hear to match what you'd like to be hearing, and that part of the job isn't too hard with a little practice. I could take a new guy and have him operationally-capable of handling a band rehearsal in about the time a weekend OW class takes these days.
Now, take that same person, and put them behind the board at a wedding (the tech dives of sound -- one mistake, and they'll remember it forever), and they might know it's a bit more toward the edge, but they could be decently comfortable. Most of the time, even in the potentially-stressful arena of wedding sound, they'd be perfectly fine. However... (and here's the big part)...
Now, if they happen to be running sound for the wedding, and 10 minutes before it starts, the bride shows up and changes the music, are they going to be able to roll with the surprises? (If the conditions change on the deep dive, do you know how to adjust?) Now, if the music was changed, and 5 minutes before it's supposed to start, an EQ leaks its magic smoke, can they diagnose the problem and handle it? Now, if they manage to reprogram the music and bodge something together around the bad EQ, and during quietest part of the ceremony, a passing trucker starts broadcasting over a bad cable *somewhere*, how long will it take to handle it?
I *NEVER* feel comfortable running sound for weddings. It's some of the most stressful sound I've ever done in my life (and that says something). I'm not scared or apprehensive or (too) terrified, but I'm constantly running through everything that could go wrong. I feel supremely confident in my abilities to handle almost anything that turns up (power outages are the hungry great whites, I suppose... nothing you can do about those), but bad things happen when you lose your edge.
I know other sound guys who are always laid back, even running major events or weddings, and I find it somewhat offensive when I see their comfort translate into poor performance. Most of the time, they do a fine job, but when the #2 hits the whirly, they fail to do the job to the quality that I require of myself.
In diving, it's not that you might give poor service to a client or disappoint a bride. You *could* die. With that card in play, although I certainly feel *competent* when I dive, and although I have *confidence* in my gear and its condition, I never want to completely "feel comfortable". I'm relaxed; I'm enjoying the dive, but I want to keep that edge to be ready to handle whatever comes along. (Or maybe I'm just odd... I skipped lunch today.)