The charge I referred to was what your phone company would charge for voice calls. I don't know about texting since I rarely use it.
Internet will depend on what wifi you get locally from the boat and hotels. Beware of using your home phone company's plan for data it can break the bank in no time.
Frankly, I turned off all data on my iPhone. I did leave voice on in case someone needed to phone me. I used my computer for anything related to data when I knew I had a wifi signal.

Have fun. You'll have a great time. Happy New Year!

Doug