I did a bit of research before we started cruising - this is the company we got a quote from:
Worldwide Travel Insurance | Seven Corners
It was about $5000 a year for a late-forties couple, as I recall. Catastrophic, high deductible (5000, I think). The cruisers that we know either have DAN, or they have no health insurance, or they use one of the catastrophic plans like the one I cited above and they pay all medical expenses out-of-pocket. We decided against getting the insurance (big chunk of change!) but we're uneasy with the decision, so probably will revisit at some point. Would be interested to hear what other full-time cruisers have decided.