The 3G iPad comes with a GPS chip inside, the wifi versions can use wifi and give an approximate position, but I don't think your going to find to many wifi sites on the water. Just adding the midfi unit doesn't really help to find your position. My advice, if want to navigate on water, take it back and get the 3G iPad with the GPS chip in it.
Or, you can just take a lot of provisions along, and a sextant, to get you through the long days while your lost at sea. And, hope someone finds you before it's to late.