While the physical activity associated with sailng will keep one fit, I believe the longevity is more a result of attitude.
That means that, IMHO, someone who sails (or any other active 'sport') would have a tendency to be of better health and condition anyway.
Un-fit couch potatoes don;t generally participate in extreme sports. A sailor would more than likely be from the active sort gene pool and naturally have a tendency to live longer.
Chicken or egg sorta debate
Better health thru sailing?
Better sailing thru health(y lifestyle)?