Few afterthoughts-
What if the underwater station is some kind of hideout from women who were brought to the island, expecting to be impregnated they heard about their fate and fled somewhere previously abandoned?
I think Juliet is okay for now. After all, she only wants to get off the island. She'd want that signal blocker turned off ASAP.
I think it's also worth mentioning that the underwater people must have some sort of contact with the outside world. What would they eat and drink? Why didn't they just cut the power cable running to the base rather than turning it off manually?
When Desmond was telling Charlie about the helicopter, it almost sounded like he was about to say "the helicopter took off, but it crashed".