I just rewatched The Candidate, then rewatched Frank's scenes towards the end. When the door blasts out at him, he has his hands up in front of him, so it's possible that may have spared him from being knocked unconscious, just knocked down. And the water being behind that door opens the possibility of being an escape route through a possible breach that caused the water to be there, does it not? And the hallway he is in when the door hits him has a blinking red light above the door, and the ladder he came down to the left of the door. I'm pretty sure as the sub is sinking, we see this same hallway underwater, and we do not see Frank there.
And I believe the plane may still be a viable option to leave the Island, and Frank is the pilot. I think the only other person who could possibly pilot the plane would be the Man in Black (who could have acquired the knowledge from Island visitors), and the Man in Black is not going to pilot the plane off the Island. And plus, Frank is incredibly badass.
So what does everyone think? Is Frank still alive or not?