Sayid thinks that he does. He brings out a diagram of The Looking Glass. He knows where the cable comes up on the beach and believes they can find The Looking Glass by following the cable.
Someone needs to swim down into the flooded station and flip a switch. It would be unlikely that someone could swim back out.
Jack says no one is going to do that, and Charlie volunteers. Jack says no.
Later, when the threat of the Others' attack looms near, Jack lets Charlie go to The Looking Glass.
When Charlie swims inside, he sees that it's not flooded, so he can breathe. He is happy to be alive. But then, two women come at him with guns.
Bonnie and Greta keep Charlie prisoner, but are later killed by Mikhail on Ben's orders. Desmond shoots Mikhail with a spear gun and Charlie goes to put the code into the keypad to turn off the jamming mechanism. Once it's off, he gets an incoming message from Penelope Widmore, but before he can do anything about it, Mikhail blows the porthole open and water rushes into the room, drowning Charlie.
