For those, who have one or both devices, you also have a chartplotter with GPS. And, it is completely wireless! There are many navigation apps available from the apps store. Some are free, others cost a few bucks.
Using the iPhone as a CP may be pushing it but the iPad has a 10 inch screen and it works great. And now, with the latest software upgrade, the iPad multitasks, too, so you can run several programs and switch back and forth.