Desmond saves Claire from the ocean and Charlie and Hurley want to know how he knew Claire was drowning. They get him drunk, but instead of telling them how he knew, Desmond tackles Charlie and says that Charlie doesn't want to know what happened when Desmond turned the failsafe key in the hatch.
Charlie keeps asking and Desmond finally admits that he wasn't saving Claire, he was saving Charlie. He'd seen Charlie getting electrocuted and he'd seen him drown.