https://www.obdev.at/littlesnitch

Little Snitch is a network bandwidth management application that monitors and controls bandwidth usage on a per application basis. It allows the user to limit and turn off certain application

Little Snitch is useful for blocking unwanted network access while at sea as well as to see what connections your computer is making to the outside world. It will not limit bandwidth per se, but will allow you to see what connections are being made and stop unwanted activities.

• Download and install Little Snitch (https://www.obdev.at/littlesnitch)
• Click on the Icon in the menu bar; click Open Little Snitch Preferences; choose Silent Mode
• Click show Network Monitor; this will show connections your computer is making. You can leave it in silent mode and watch the connections or choose to deny or allow the different applications.

A good link to get started: https://www.obdev.at/products/littlesnitch/getting-started.html

It is best to test this before heading out on your cruise and become familiar with the how it works.

Here is an example of the connections defined for an At-Sea profile based on 2 days of activity on one mac:

Most of the deny statements (red dots) are applications running in the background trying to check for software updates.