Boeing 777 aircraft are safe to continue flying; the question is what caused the aircraft to fly until fuel exhaustion in the southern Indian Ocean (assuming that it did in fact go into the southern Indian Ocean). At this point I feel a lot of folks are hoping it crashed in the southern IO and the large search effort is to put minds at rest.

As for submarines -- there may be subs involved in the search, but I doubt it. It's a long way out of the way for assets that are already heavily tasked. But like I said, IMO peeps really do want to find this aircraft, just to be sure.