When I am outside of wifi and cell coverage, or even by choice, how can you use a third party GPS?
From the frequently asked questions page, it appears Navionics has not tested third party GPS devices:
Q. If I attach an external GPS to my iPod Touch 2nd Generation, will I locate my position on the Navionics app?
A. Navionics has not tested this thoroughly enough to guarantee a good performance.
The answer suggests third party GPS devices should just work, but may not be reliable.
You might want to look in to Bad Elf Pro. It connects to the iPad/iPhone using Bluetooth.
Here is a link I found: http://store.fugawi.com/products/bad-elf-gps-pro-for-ipod-touch-iphone-ipad