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It was the Google map app that used to come with iPhones and iPads.

To expand a bit on FlyingDiver's answer:

What you needed on your Mexican cruise was an offline map app that included actual maps. You might be surprised to learn that the three most popular iPhone map apps (Apple Maps, Google Maps, and Waze) don't actually have any maps built-in!

Why don't they have the maps built-in? Two reasons:
#1: Maps are being updated constantly. There are probably thousands of changes made daily to maps.
#2: Because a map database of the entire world won't fit on your phone. One such database, OpenStreetMap, is over 400GB in size! The data is highly compressible, but even compressed it's about 30GB.

So, if they don't have any maps built-in, how do they work? The Google map you used works like this:

1. The app requests your location (latitude and longitude) from your iPhone.
2. Your iPhone tells the app your exact location (which it receives directly from GPS satellites.)
3. The app the sends that location data (over the internet) to Google's servers, and finally
4. Those servers then send a small section of the worldwide map — perhaps 1 square mile — centered on your current location.

Steps 1 through 4 repeat as long as you're moving. Steps 3 and 4 require an internet connection — either Wi-Fi or cellular data. The app simply won't work if you don't have an internet connection. Apple Maps and Waze work the same way, except that they connect to Apple servers and Waze servers*, respectively.

If you go to the App Store and search for "offline maps," you'll find lots of maps that work without any internet connection. They manage this trick by using highly compressed maps, and by requiring you to download maps for a specific area in advance. For example, before your trip to Mexico, you would have had to download the Mexico map to your phone. You can have several downloaded maps installed at once, but you are limited by the storage capacity of your phone.

Here's a roundup of offline map apps:

* Google bought Waze last year, so eventually Waze will probably switch to using Google map servers. But right now Waze still uses it's own servers.
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