How do I get from the zone that is underwater back to Orgrimmar?
Easiest and fastest way? Just use your Hearthstone.
Alternately, there are one or two above-water flightpoints from which you can hop on a windrider to Undercity or Grom'Gol and catch a Zeppelin from either of those. But they are phased, so you likely won't reach them until you've done several hours worth of quests in the zone.