Here is what you should do:
Upgrade your iPhone 6 to iOS 9.3
After that is done, go to settings/phone and turn on Wifi calling. You will be asked to enter a 911 address - just put your home address.
No when you get on the plan to go to the Caribbean, put your phone in airplane mode and do not turn off airplane mode until you are back in the USA.
When you get to your destination, turn on ONLY wifi. Now whenever you are connected to a free wifi point, you will have full access to your phone - internet, calls, text. Calls to and from the USA will be free. No charge for texts. Internet is on wifi (actually everything is going over wifi). Calls to local places at your destination would be charged as international calls as if you were in the US, so you may not want to do that.
Enjoy your trip. The ability to do this only came into place this past Monday!! Good timing for you.