Hi,
Behave about something :
1) iPads what are only wifi models don't have a real built-in GPS, but a A-GPS (Assisted GPS) what is not enough accurate to navigate with aboard.
2) iPads what are wifi AND cellular have a real built-in GPS (similar our GPS chart plotters) what perfectly works EVEN if there is no SIM card installed (so, no need of plan contract) and perfectly works aboard boats, cars, planes.
On my boat, I'm using a iPad 3 wifi / cellular without SIM card installed, and it perfectly works with Navionics maps. Once again no SIM card, no plan, no contract.
For those who have only wifi Ipad (no cellular), forget any Bluetooth dongle. Buy the following add :
http://www.amazon.com/Bad-Elf-Connector-iPhone-touch/dp/B0035Y7ZJ2
Bad Elf GPS works perfectly on only wifi iPad, and iPhone as well. I bough it years ago to use on my former iPad wifi and my iPhone, it's great and not expensive. So, you don't need to buy a wifi and cellular iPad of you already own a only wifi iPad.
I remember using my former iPad (only wifi) with Bad Elf GPS connected, aboard a Airbus 320, flying from Paris to Greece. My seat was window side and the iPad showed perfectly the speed, altitude, of the flight.
Hopefully you understand what I meant, even with my basic English, sorry about that !
For more information, you can PM me on TF.