Email is done via wifi, so I don't know why you would be charged extra for any "data" from an email. You can do several things over wifi while your phone is on airplane mode that do not eat up your data, including sending emails, surfing the internet, streaming, emails, imessages, etc. To my knowledge you should be good to email as much as you want without having to worry about a data usage charge from your carrier, as long as you are connected to the ship's wifi.