Jim, you can quantify many of the risks but I suspect this is a little bit like the saying that insurance companies charge more for red cars. It isn't that red is somehow more dangerous (if anything it is easier to spot) but that the folks who pick flashy cars probably also are "flashy" drivers, i.e. more aggressive, and that's what is being targeted.
A lot of cruisers frankly cruise solo or short handed and don't keep an adequate watch. A lot of ocean freight doesn't keep watch either. And, folks who go to sea in small boats are likely to be less conservative than the ones who stay on land. (Wm. Buckley Jr. aside.)
So I'm not sure you could sort out all the issues and just use "ocean voyaging" as a sole criteria. I think skill and experience levels, age, occupation, and others would be the weighted factors, not just voyaging.
Are voyagers also non-smokers? In that case, maybe that's all an insurer needs to know. One big set of bills they won't pay, and if they disappear at sea, you've got seven years before you have to pay out anyway.