Bill:
Odds are, you and your wife drive, know of a number of people who died in vehicle wrecks, some of whom were better drivers than either of you, and yet odds are you guys still drive, and not just when you have to.
So many people dive, and the deaths don't seem real common, and you have so much control over your own risk mitigation, that it seems diving is considered 'safe enough,' which is about all I can say about any hobby.
I took a look at your profile; you haven't done Rescue Diver yet, have you. I think you guys would benefit from that course, and perhaps even by following up with the SDI Solo class, even if you never dive solo in your life. Here's why:
1.) Both courses teach you a pro-active risk mitigation mindset; methodical pre-dive gear checking, and being mindful of risk assessment.
2.) Both courses teach you, in different words, that there's a lot that can kill you diving, but very little that should. Usually death is the product of multiplying an unforeseen event by a bad reaction. It's said a number of dead divers are found with their weights not ditched, for example. Panic, not great white sharks, is the great predator swimming the seas looking for divers. Learning about panic, and developing the knowledge base, mindfulness and confidence to abort that reaction and follow the mandate to Stop, Think, Act rather than Freak Out is the key to survival many times.
3.) Ask yourself; what in diving do you think is actually likely to kill you, if you are a thinking diver who doesn't take stupid chances? Everybody makes mistakes; not everybody is reckless, though.
4.) As to the consequences of a heart attack underwater, yeah, that could be bad. Almost as bad as flying down the Interstate at 70 mph, getting hit with sudden extreme pain, and swerving into the other lane and killing yourself, your passenger(s), and possibly somebody else in another car.
5.) Seems to me you're conscientious and mindful of risk, not overconfident and cocky. That also should drop your risk.
6.) I think sticking to viz. 10'+ is quite reasonable. Anything less, and being solo trained seems like a good idea, anyway.
Richard.