Sat phone would be the only real option. These rates would about 10 times what you pay for roaming cell phone
charges. A Verizon setup might work a lot of the time along the east coast
of the US in the ICW
for example but even then not 90%. It too won't go south with Internet coverage. That is not a commonly available thing.
I would contact Sprint and see where they actually have coverage. It won't be most places but you might have in a few ports
on the northern most leg.
Chuck indicates what is pretty standard for most people. WiFi
hot spots with broadband
are getting almost very common in most places.