what is the purpose of the soaking water?

The soak is meant to clear Cole's mind of anger. In Chapter Seventeen, Edwin declares that they will go swimming in a freezing pond to teach Cole a lesson. He makes Cole dip completely into the water shoulder-high and then asks him to break a stick, whose left side represents anger and right side represents happiness. The lesson implied is that if you focus on anger and try to break the left side off, a left side of the stick always remains. Edwin tells Cole how when he was banished to the island he would dip himself everyday in the freezing pond and try to focus on the happy end of the stick, not the angry end.