I would guess there is some significant caloric expenditures when diving regularly. The heat you lose has to be replaced. How much that is would of course depend on the conditions you're diving, but it should apply to nearly everyone.
Add in lugging gear, exposure to sun and fresh air, mental health benefits, inducement to refrain from or stop smoking, inducement to engage in cardio exercise apart from diving, and probably a few other things, i wouldn't be surprised to find that long-time regular divers are on average more healthy than the general population.
And on a purely speculative note, it's known that hyperbaric treatment is beneficial in many ways, and it's easy to think that the same benefits might be had from scuba diving.
