The iPad without cellular connection does not have GPS onboard, so you cannot use it reliably for navigation. It does have very limited location awareness when it comes in range of Wi-Fi hotspots that have been cataloged into the iOS location database and also falls in the subset of this database that gets loaded onto your device for offline use.
The maps themselves are designed to function fully without a data connection, so although you might not get updates or supplemental data while the iPad data connection is offline, an iPad models with cellular radios and GPS chips will work well for navigation apps such as Navionics even when you are out of range of all Wi-Fi and cellular towers due to the design of the software, the OS and the GPS chips embedded in the iPad.
