iOS Updates when bandwidth is restricted
Regarding the OS update, I seem to remember that filling the iPhone with (songs, books, pictures, apps) to the point where the OS update would not fit was the only way to prevent the OS update from downloading.  On a cruise, where you would likely be taking many pictures, etc, this is not a very amenable solution.

When I've had metered WiFi in the past, I would turn it off except when checking mail and then turn it back off immediately after downloading all the messages.  Depending on how fast your WiFi connection is, even this may or may not have prevented the update from downloading.

About the only thing remaining is to ask the network admin onboard the ship if they could block the software update site as if it were a torrent or porn site.