iOS, Cruise Ships and data usage

I was on a cruise ship in the Caribbean a month ago and needed to keep in touch via email.

Connect@sea offered a 300mb plan which I thought would work. I followed all of their guidelines about what to disable etc.

All was going well until...

I woke up on day 3 and IOS 8.1.1 had downloaded (182mb) and was ready to install. After the 182 mb download, my wifi package was also empty.

I can't find any way that I could have prevented that large automatic download. Is there a way? (It's true that it doesn't install until you approve it, but it downloads automatically.)

What are the best practices when you take your iPhone with you on a cruise? (it wasn't easy at all actually, there are a lot of controls to use when you don't want to use cellular data, but everything is designed to assume that WIFI is a giant unmetered pipe---not a restricted connection that you don't want to suck data from.)