It's a little more complex than just saying some body of water, it has to do with how far you are from an emergency landing strip at any given time. As long as your flight across the atlantic did some form aereal island hopping, you're good. Obviously, if you have to keep jogging north and south between landing strip zones, then you're covering more distance than a flight that could just take a direct route without regard for proximity to landing strips. ETOPS was a compromise to let twin engine aircraft remain competitive. You can read a little more about it here:

http://www.boeing.com/news/releases/...31015gq&a.html