If your boat has WiFi you can use WiFi calling and it’s free. The other option is to activate the daily international plan with AT&T.. If I recall data and texts are free and 25 cents a minute on calls with the plan. Incoming calls free. 10.00 a day is the cost. It’s been a bit since I had AT&T
You will also be in range of the AT&T towers in the USVI when in the western part of the BVI. If you lock out roaming and get good cellular your in network!