1, i may be wrong here, but im thinking the hatch a some kind of teleporter.
you only ever see people come out of the sub. i think they are teleported to the hatch, then taken to the island by the sub.
this would explain juliet being drugged first.
2, maybe the reason charlie didnt die is because they went a day earlier than planed because the others are coming,
OR
des only said you press the button then drown, and charlie hasnt pressed it yet.
maybe when he does it courses the hatch to self destruct trapping charlie.