Conflicts between your iPhone and the ship's wifi:
On a recent cruise I was having a hard time getting my phone to work both as a phone and as a wifi hotspot for my laptop, even though the phone was showing a cellphone signal. After asking a large number of crew I finally found out that my iPhone was being switched by the ship's stronger wifi signal to the ship's wifi rather than the normal cell network. The iPhone has an option to tell it to ignore any particular wifi signal so I blocked out the ship's wifi and was then able to use my phone both as a phone and as a wifi hotspot.