This will work on iPhone, iPad & Android mobile devices running an up-to-date version of Google maps. To make a map available for offline viewing just search for your desired location and get it up on your screen so that it only shows the area you want to save. If you zoom out too far you may get an error asking you to zoom in before it can be saved, but you should be ok so long as you are only trying to capture Kefalonia Island not the whole of Greece!
Or you can test it by saving a copy of your home town area & putting your phone into 'airplane mode' to disable data & wifi connections.
The first method is to just put 'ok maps' into the search bar and press search. This will automatically start a download of the map to your cache.
The second method is similar: once on your location, tap on the search box again and scroll down to the bottom. Here, you'll find an option to download the map offline and once you tap on it, will download it to your cache as well. Once either of these methods have been done, you will have an offline version of your map that you can use whenever you want and supports GPS to pinpoint your position. The main features that you will not be able to use are the search facility and the ability to get directions.
When your holiday is over you can clear up the space used on your device By going to
SETTINGS > APPS > MAPS and clearing the cache. As you can see it's surprisingly efficient and only takes up a small amount of memory so this step is not essential.