First we had Daniel in Black whose association with water meant that he was willing to change, evolve, and become human. He embraced humanity and saw the Fall as an opportunity.
Here, in Reichenbach, we also have Castiel associated with water in an episode where he learns from past mistakes and learns to let go and well, if not let God, then let matters fall as they may.
Water has become a symbol for going with the flow this season. I hope we see Dean near water when he overcomes the Mark of Cain. Water is purifying and cleansing. It would be only poetic that he be baptized and reborn near water.