The future (Ipad w/ Navionics)

EXCEPT cellphone service is virtually non-existant for at least half of the places you would need to use navigation in a yacht in the Bahamas.

If you want to seriously interface anything as a backup that's reliable for navigation anywhere in the world. Interface a Garmin handheld to a laptop with the charts installed on the laptop. Then you have a real GPS, transmitting the data.

Personally I navigate the USCG approved way. I plot my courses on paper charts, put the waypoints into my handheld garmin, and create a route. I use whichever boats chartplotter (that I am on) as a guide. Needless to say my routes stay in my handheld and I use them on subsequent trips on other boats. BUT, I always have a backup handheld AND paper charts for wherever I am cruising on me.

iPad navigation

I used my iPad from Fla to the Bahamas on the boat and have used it all around the Abaco's these last two Months with no Cell service. GPS worked worked great all over the Bahamas.