Lucas had been trying to tell them what happened the entire time and no one had caught it but him.
Wasn't Sam said to be the smart one? Granted, he had been hiding his intelligence all his life, per Death's request, but still.
At least Sam realized that the ghost killing the people was Peter Sweeney.
But how do you get rid of a ghost when the body isn't there?
At least Jake figured it out, at the cost of his own life.
Lucas was freezing from the lake though and Dean had to use his flames again, this time to push the water from the boy's lungs and to keep him warm till they reached the shore.
At least he started talking again.