This post is quite old, probably a floater but I am also keen to find such a contraption for my iPad.
GreyMatters—
I understand where your thinking comes from, your moniker should be BrownMatters. Smug; Supercilious; Yet so asinine.

iPad wifi only cannot receive GPS signals.

iPad wifi + 3G can receive GPS signals. For $120 more, 3G and an independent GPS are included. Two excellent marine navigation apps are Navionics and INavx. All data is loaded onto the device so a GPS fix is all that is required, no Internet needed. It works very well, I’ve used it in Panama (the country, not Florida). Certainly no wifi there, a bit further than 10 ft from the nearest starbbucks. Will post if I find anything that is suitable.