The first and most important point is coverage. By all means take a look at the coverage maps supplied by the phone companies, but the bottom line at this writing is that there are really only two choices within the United States: AT&T or Verizon. Other services, like Sprint and T-Mobile, may be fine in your home area or if you travel along major highways, but once you start to cruise further afield you will find that only Verizon and AT&T offer the type of coverage you need. Even with one of these carriers, I suspect you will still encounter some dead spots if you are traveling south down the coast to Florida.
This doesn't mean you have to have a long-term contract with AT&T or Verizon. I personally have been using various MVNOs operating on the AT&T network for many years, and I have found coverage excellent from the Florida Keys to coastal Maine. MVNOs use the same towers, the same frequencies, and generally the same signal that you would get if you were a contract customer with AT&T or Verizon, but until very recently you were not able to utilize the latest and speediest LTE data services. Within the past week or so that changed with MVNOs Net10 and Straight Talk (both run by America Movil) beginning to provide access to LTE data for appropriately equipped phones. The speed gains reported by users have been dramatic.
Keep in mind that while cruising coastal waters far from major population centers you might very well be out of LTE range, or even 3G data range. Don't count on getting the same data speeds, or even any data. In other words, it is not a wise idea to run programs or apps that require a constant data connection, even if you can afford to pay for it. Most charting programs, for example, allow you to download and store charts on your phone or other device, rather than accessing charts directly from the Internet.
If you are already under contract with AT&T or Verizon it might be time to start thinking about the most economical way to end the arrangement, especially if you are planning on leaving the United States. It wouldn't make any sense to keep paying hundreds of dollars a month for phones and service you can't use. In some cases, particularly if you have GSM phones that utilize SIM cards, you might be able to purchase a local SIM card and phone service when in another country. However, my own two cents is that it is usually cheap and easy to purchase a local phone and service anyplace you are staying long enough to need a phone. I personally don't like to carry around flashy and expensive phones or anything else when in many places, but a simple and tiny flip phone or candy bar phone can slip right into your pocket and won't make you cry if it is lost, stolen, or broken.
I'll have more thoughts on mobile phones for cruising later, but for more up-to-date information on mobile phones and carriers check out Howard Forums and prepaidphonenews.