Selecting an iPad for navigation is not as complicated as some say. Any iPad will work. The major difference is whether or not an external GPS receiver is required.
All iPads with cellular data functionality have a built-in GPS receiver. Apple denotes these iPads as WiFi+Cellular, WiFi+3G, (update: WiFi+4G). No additional hardware is required to use these iPads for navigation, although software is required (more on that later).
No WiFi-only iPads have a built-in GPS receiver. Apple simply denotes these iPads as WiFi. To use these iPads for navigation, an external GPS receiver is required. The Dual Electronics XGPS150A Multipurpose Universal Bluetooth GPS Receiver with Wide Area Augmentation System and Portable Attachment works well for this purpose.
Using an external GPS introduces some inconvenience. Although it has roughly the same usage time as an iPad, it is another device to charge. In additional all Bluetooth devices occasionally have difficulty reconnecting, and must be forgotten in the iPad’s Settings and then rediscovered.
There is negligible cost difference between purchasing an iPad with integrated GPS receiver and purchasing a WiFi only iPad and an external GPS receiver. Therefore if purchasing an iPad primarily for navigation, an iPad with cellular functionality is the clear choice. However, if one already owns a WiFi-only iPad, then spending $85 for the external GPS receiver may be a better option.
Aside from the built-in or external GPS consideration, all other considerations are personal. The smallest amount of storage (16GB) works fine for navigation. Display size is a balance between portability with readability. There are some differences in display brightness and contrast ratio worth noting: more brightness and higher contrast ratio make for better readability in direct sunlight.
And of course there are price point and intended use to consider. Will the iPad only be used on the boat, i.e. occasionally? Will it be dedicated to navigation? The author prefers to use his “otherwise retired” WiFi+3G model with an integrated GPS receiver. No tears will be lost if it goes in the drink or someone spills their drink on it.
Remember that in additional to the iPad, a navigation app is required. There are several navigation apps in the App Store available for direct download to the iPad. (Note that these apps do not appear in the App Store on an Apple computer, but only in the App Store on an iPad or iPhone.) The author prefers the Navionics+ apps.
Incidentally, any iPhone will work for navigation in the same way. All iPhones have a built-in GPS receiver and have screens designed to be visible in direct sunlight. One drawback of course is the screens are smaller. However, phones easily fit in pockets and cockpit cubbies.