Can you use iMessages on the ship between iPhone users

without wifi?

If not will the basic internet plan allow you to send iMesages to other iPhone users on the ship? What about those with iPhones back home?

you have to have an internet plan but it does work with all phones. You may have occasional delays.

I believe we discussed this before. You will need the Social Media Plan for each iPhone you want to be able to use iMessage for if you want them to be able to be on at the same time. You cannot use iMessage without wi-fi or cellular data. It will work for both iPhones on the ship and elsewhere.