My wife and I dive together EVERYTIME we dive. Our youngest is 10 so I understand you wifes concern. But, life is too short to play the "what if" game. The likelyhood that you would both die in a scuba accident are so slim as to not even be a concern IMHO.
I think the likelyhood of two people on the same dive dying in a scuba accident is slim let alone that the two people are husband and wife. Also, too many people play the what-if game only halfway, focusing only on the negative. Let's play the other half. What if both of you have a wonderful time together scuba diving without incident and through the shared experiences come closer together?