Good thing there's scuba gear onboard and someone who knows how to use it.
How was the guy in the plane alive, since it's been at least a week+ since the place went down, right? Some time passed during the military quarantine, days at least and it's been days at sea.
We are to assume Alex offed the other guy on the boat after he tried something. Not that he was wrong, kid could die at any minute and turn into an undead. Which was why Strand was also kinda right.
The name of the episode is ouroboros, the symbol of the serpent eating itself. So that means...something.