We have had health insurance all the time we've been cruising.
The first year we had Blue Cross/Blue Shield, and when I needed x-rays and stitches in the Bahamas, BC/BS did not reimburse me for my expenses.
We finally found International Health Insurance/danmark. www.ihi.com
When I had cancer, it covered astonishing expenses, and once you have been accepted for the insurance you can't be cancelled. They do not accept applications from US residents (we were not residing in the US so it was not a problem for us, though most of my cancer treatment was in the US), but you can be a US citizen, provided you reside outside the US.
IHI premiums are based upon age and size of deductible, and there are various plans. You can get a quote on line from them, www.ihi.com
They usually will not quote to those living and based in the US. A US citizen living elsewhere is no problem, however.
Sample premiums (I only have a 2005 booklet, so will be higher):
Age: 40-49 - Ann. prem. $3,939 w/ $500 ded., $2,083 w/ $2,000 deductible, $1,632 w/ $5,000 ded.
Age: 50-59 - Annual premium $5,523 w/ $500 deductible, $3,065 w/ $2,000 deductible, $2,391 with $5,000 deductible.
Age: 60-64 - Annual premium $7,575 w/ $500 deductible, $4,118 w/ $2,000 ded., $3,365 w/ $5,000 ded.
There are maximums paid for various categories such as surgeons, hospital rooms, whatever. Peter's thyroid surgery in Australia was completely covered by IHI. The ridiculously high hospital room rates charged in New York were not, as were most other hospital/medical fees.
I've been quite happy with them.