It seems to me that being aground at least meant the ship didn't sink below the surface, so they had as long as they needed to evacuate.
Indeed, it seems fortuitous that the Concordia hit bottom where it did because it could have easily sunk completely or capsized.
I saw a computer graphic on the evening news showing the rather steep sea shelf that the Concordia is resting on now. Waves and the current could easily knock it loose, causing it to slide down this shelf into much deeper water, not to mention roll over repeatedly on its way down. That would likely be lethal to any rescue/recovery personnel onboard.
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