Microsoft Windows

Disable wifi when you are not using it
Prior to leaving port you can download & purchase software like NetLimiter or NetBalancer or TMeter which will allow you to monitor and block network/internet connections.
Prior to leaving port check out these browser recommendations such as Ublock Origin or Adblock Plus as well as an image blocking extension/add-on, advanced users may want to try uMatrix.
Disable Dropbox, Weather Bug and any other resident service that shows up in your taskbar.
Disable Windows updates by browsing to Control Panel > System and Security > Windows Update > Change settings to change to Ask before downloading
Disable antivirus automatic updates (varies by provider)
Windows10: While connected to wireless, Settings app > Network & Internet > Wi-Fi > Advanced Options > Enable the “Set as Metered Connection” slider here
Windows10: Start > Network & Internet > Data Usage
Windows10: Update & Security > Windows Update > Advanced Options > Choose How Updates are Delivered > disable Windows10: Settings > Disable “Update Apps Automatically”

Apple Mac OS

Disable wifi when you are not using it
Prior to leaving port, you can download & purchase software like Private Eye or Little Snitch so you can audit (and block in Little Snitch's case) what software is connecting to the network/internet.
Prior to leaving port check out these browser recommendations such as Ublock Origin or Adblock Plus as well as an image blocking extension/add-on, advanced users may want to try uMatrix.
Disable Dropbox, iCloud, Weather Bug and any other resident service that shows up in your top bar.
Disable App Store automatic updates by bringing up System Preferences > App Store > uncheck Automatically Check for updates
Disable antivirus automatic updates (varies by provider)
During the cruise, you can use the Network tab in Activity Monitor to view individual program internet usage.

Disable wifi when you are not using it
Prior to leaving port download monitoring app like DataMan Next, smartsapp, etc.
Prior to leaving port consider lightweight browsers like Opera Mini or Text Browser<\li>
Keep apps closed, as some will actively try and update/download when they are open.
Settings > iTunes & app store > Disable non-essential apps (make sure not in low power mode to access option)
Settings > general > Disable background refresh
Settings > iCloud > disable icloud for non-essential updates (specifically pictures and drive) OR settings > user > iCloud > disable non-essential apps
Settings > podcasts > disable podcast downloads
Settings > news > disable podcast downloads
Settings > Mail > accounts > Disable fetch new data
Settings > Mail > disable Load Remote Images
Settings > Messages > Low Quality Image Mode (for iMessage)
Chrome > Settings > Data Saver
Facebook - Disable videos autoplay: More > Settings > Account Settings > Videos and Photos > Video Settings > Auto-play > Never Auto-plays Videos
Twitter - Disable videos autoplay: Gear > Settings > Data > Video Autoplay > Never play videos automatically
Instagram - Disable videos autoplay: Gear > Mobile Data Use > Use Less Data

Disable wifi when you are not using it
Prior to leaving port download monitoring software like My Data Manager or Data Usage Prior to leaving port consider lightweight browsers like Opera Mini or TextOnly
Disable autosync: Settings > Accounts > More Settings button in the upper right corner (represented by three dots) > Uncheck Auto-Sync data (Manual sync by tapping the accounts and selecting the module)
Settings > Data usage > Data saver
Turn on and uncheck non-essential programs
For a guide check out this PDF Use Less mobile data with Data Saver from Google. Enable Network Restrictions on “RVIB_NBP” to treat it like cellular data.
Settings > Data usage > Network restrictions
Settings > Data usage > Wi-Fi data usage > Select app > Disable background data
Settings > Data usage > Wi-Fi data usage > Select app > Disable Unrestricted data usage (also enabled by data saver)