Don't forget, a GSM network has more capabilities than pure phone
service, for instance internet and email access. There are global
providers mostl with cheaper roaming fees than you US-provider is
asking for, but whenever we stay a little longer in any country, just
go for a local SIM-card. Internet access via GPRS or UMTS is sometimes
surprisingly cheap (sometimes not), and I prefer to make my bank
transactions in the privacy of my own boat.

What Phone: GSM only is worldwide :

be sure it is a tri-, better quadband GSM as US is using other
frequencies than the rest of the world.

if money permits, be sure it is at least GPRS capable, better though
EDGE best UMTS

UMTS, in some countries called 3G, uses different technology and is
much faster, but mostly only available in larger cities, but this
depends on the provider, might sometimes be countrywide if it is a new
proider without the "old" technology. All UMTS phones are GSM capable
too. This comes closest to broadband you might be used at home.

EDGE is an enhancement for GSM, cheaper upgrade for the providers,
they do not need new Antenna towers, just a few electronic parts, so
it is more widespread, but much faster than GPRS, much much faster
than ISDN

GPRS was the first data service for GSM phones, still much better then
using dial-up service.

All these data services are usually volume paid, if time, then onl the
actual transmitting time, much better again compared to dial up. One
thing I agree, learning how to program cellphone and computer to work
together might sometimes be frustrating.

for the technical terms wikipedia and/or searchmachines are
helpful ;-)

Still I'd have an external WIFI antenna on board just in case you
catch a free signal somewhere, marinas and restaurants sometimes
provide free service, sometimes for a charge. ;-)