We've been doing the ICW migration since 2008. WiFi, as others have indicated, is about extinct. You may be able to get WiFi in marinas, our experience is that marina WiFi is essentially useless. There are some that have updated to OnSpot, that seems to be universally usable.
Our solution is a cellular amp and a 4G modem. It connects to our Cradlepoint router to give us the flexibility to use marina WiFi via a Bullet HP radio when it's useful, and underway, we have generally uninterrupted coverage via 4G/LTE. Our gear is now 4 yrs old therefore obsolete, but still functions perfectly. We use a Wilson Sleek 4G cradle amp with external antenna, it allows us to use the phone, modem, etc. and is effective to the extent that it makes the difference between no signal and being able to use cellular. Would not be without it. It is HUGELY effective here in the Bahamas; our BTC coverage here has been great and far better than WiFi. $35 for 15 GB per month for LTE. For those planning travel to Bahamas, get an UNLOCKED device, BTC has on their website a pretty comprehensive guide to what devices will work with their 4G network. It is possible to buy an unlocked iOS device that will work with Verizon in the US as well as in the Bahamas with a BTC sim card, but you can't purchase it through Verizon. YMMV.
The cellular amp solution offers several alternatives many of which have been discussed on other threads. Generally, the repeater amps designed for automotive use are problemmatic aboard, the cradle style requires the device be placed in the cradle, but avoids the feedback issues of the repeaters. Our Verizon 4G/LTE modem does very well with the cradle amp, we typically have coverage on most of the east coast and offshore out several miles, depending on location. There are some dead spots in the boonies of the Carolina swamps, but coverage is nearly continuous on the ICW.