Had a look at G-Fi. Not really suitable for a permanent installation on a boat. Battery life of 5 hours and unlikely to be waterproof and mountable in the marine environment.
Looks like I shall have to continue with alternate plans. Bad Elf looks best in aquapac. Unless the wireless route works, using my iPad as a cockpit repeater for the permanently installed navigation system.
If you get the Bad Elf tell us how it works. I'm thinking of getting one. It looks extremely small. I wonder what it uses for a power supply.