We've been on a few cruises, including two World ones, and are going on a third next Jan-April. We understand the cost to be so high cos of the fact that you are often in the middle of the ocean when you might fall ill and that means a helicopter to the nearest hospital, which could be a very long way off. The nearest hospital might not even be on your booked route, so I imagine getting you back on board (if that is possible) would also be prohibitive.
We switched to Nationwide a few months ago and their quote for our upcoming cruise is just over 1/3 of what we paid for the last World Cruise, so well worth the monthly fee of £19 for the bank account. I have had two TIAs and suffer from Atrial Fibrillation, DH suffers from Peripheral Neuropathy. DH will be 79 when we travel (me a mere 74) and I believe they won't cover him after he is 80.
We have used Staysure in the past, with competitive rates, but not since we developed the latest two health problems.