Either that or they're used to entering potentially fatal hostile environments and just don't think about it, of course. We see enough of that on the roads.
Anyway, I've no idea what a "try-dive evening" involves, but it sounds a long way from qualifications needed to dive at sea with someone you might end up needing to rescue. If it could be done safely with a non-swimmer, it might be the inspiration they need to go away and acquire some proper swimming skills.
(And I second Legs' point about surface swimming vs diving. In my anorexic pre-teenage years, I was negatively buoyant (sitting cross-legged on the bottom of the pool sort of thing), and swimming within breathing range of the surface consumed vast amounts of energy compared to blatting along underwater, which I'd do to great effect. I've not done much swimming as an adult, but what was noticeable was that in the intervening years I'd acquired the ability to *float*, opening up all sorts of intriguing possibilities.
I'd love to have a go at SCUBA, but I'm severely out of practice with swimming, and my middle ears don't react well to rapid changes in pressure.)