I have turned one on in the middle of the Atlantic and left it on till it went flat and nothing.
If you wade through all the crap it's an assisted gps.
It will narrow it down through cell triangulation then the gps should bring it down further. But if it can't do the first it won't usually do the second.
I have a iPhone 3GS a 4 and a iPad. None will pick up a signal offshore.
Rob