Flashes Before Your Eyes - S3-E8
Corrected entry: Desmond claims that he was saving Charlie's life by saving Claire from drowning by jumping in before he does. However, if Desmond had not had a flash to witness the event of him dying, Charlie wouldn't have known it was happening either as he followed Desmond to the event. As we find out later, they were a mile inland, so there's no way Charlie could have found out until a long time after Claire had drowned.
Correction: Incorrect. After they learned that Eko was killed, they would have returned to the beach, where Claire was drowning. Then Charlie would have died trying to save her.