I think it depends on where you plan to cruise, though I recommend health insurance for everyone. You never know what is going to happen. A few anecdotes, then more comments.
A woman was in Sri Lanka, needed emergency surgery. She was flown to Singapore for treatment, did well. Singapore medical care is excellent and significantly less expensive than the U.S., but for those with government health care programs, it is not so cheap.
A Canadian friend had her big toe almost severed in an accident with an outboard while they were in the Bahamas. She flew back to Canada for treatment.
See my cruising dictionary, "Medical Information" at http://www.cruiser.co.za/faq5.asp
and "Hospitals", http://www.cruiser.co.za/faq3.asp
We left the U.S. in 1986 and after several years of cruising knew that we were not going to return for the foreseeable future, and thus had to find more reasonable health insurance than what we were paying a ridiculously high amount of money for and which was pretty useless for treatment outside of the U.S.
We found International Health Insurance/danmark, a Danish insurance group that specializes in overseas health coverage. If you live and cruise in the U.S. it will not be as good because US health care is significantly higher than what will be paid for by IHI. But in general it will cover most overseas medical care 100% (after the deductible is met). They paid out a huge amount for my cancer treatment in 1996, which was done in the US (and so did we). Had we had to pay the entire bill out of our own pockets our cruising days would have abruptly ended.
There are "travel" health insurance programs for those who will not be traveling far from their home shores - I remember that a Canada insurer had an excellent program, and I've heard through SSCA (Seven Seas Cruising Association - http://www.ssca.org)
of some that cover U.S. citizens.