My understanding agrees with that of JRP. The service providers have worked to steer their coverage where the customers are. Several years ago I could watch streaming video on my laptop while sailing from Annapolis to Solomons. Now the signal is down significantly out in the Chesapeake. Interestingly I had a great AT&T 3G signal from Block Island to City Island a few weeks ago.
That said I've had great luck using a laptop/datacard and/or my iPhone for weather updates, local information, and e-mail in anchorages up and down most of the East Coast EXCEPT for North Carolina which seems to be a cell phone wasteland.
For iPhone apps there is Navimatics with the entire ActiveCaptain local knowledge database. $20 / coast.
sail fast and eat well, dave
beware "cut and paste" sailors.
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