This is an incredibly complicated question. I am not a tax or insurance advisor, but have been a physician for a long time, am a cruiser, and a US Citizen but also an EU resident. Below are observations I've made professionally and while cruising. It's such a complicated question, that everyone must eventually investigate and then decide what best suits their situation.
I assume you’re a US citizen AND not officially a resident of foreign country. I don’t know how old you are. There are several things to consider.
Every year, I see cruisers who develop unexpected medical problems. I've had them myself. A cruising friend developed a collapsed lung near Somoa. He required emergent transport by ship to Apia-a very expensive trip. In Apia, he received life-saving treatment, but couldn't leave there as ANZ refused boarding even when he met their demands. He almost died but was eventually medevac'd via private air ambulance for almost $100K. Most air ambulance companies will not transport without being relatively sure that they'll be paid.
Medicare does NOT cover anything overseas, including MedEvac. If you get sick abroad and have MediCare, it would make sense to go back to the US or its territories. To make matters worse, even though MediCare doesn't cover non-US care, you may still be required to pay part A MediCare payments if you're close to age 66. Medical care, as Bill notes, will be much less expensive abroad, but may or may not be comparable. AU, NZ, SGP, KUL, BKK, ZA, HKG, and some places in India, have really good care; in many places, it can be scary.
Since you have little control over where you'll need care, coverage is preferable even if you're healthy.
Some US carriers provide international coverage, others don't. My employee BCBS did, but our UHC policy did not.
You also need to consider the Patient Protection and Affordable Care Act (PPACA). It requires most US citizens to have health insurance or pay a penalty, starting modestly in 2014 and increasing in subsequent years. If you are a resident of a foreign country (meet the IRS test for a foreign tax home), then you are exempt from the PPACA. Check with a tax advisor or the IRS. If you live in a state with ObamaCare, you might look into coverage with an exchange; coverage and price vary greatly; not available in most 'Red' states. A high deductable exchange plan and MedEvac insurance might be a good package.
An alternative is to get MedEvac insurance only for a few hundred $'s annually. That way, you could get back to the US if you have US coverage, or somewhere competent and cheaper than the US if not.
Most EU and 1st world countries have coverage available for their residents (don't necessarily have to be a citizen-it's complicated and varies by country.) If you're cruising, then in-country coverage typically won't cover you. Most of my EU colleagues have some type of coverage.
Sorry this is so complicated. It's bad in the US, but worse for Americans abroad. Good luck.
SY Revelation SM390