How to stop my horse from rolling while I am on him?

It all comes down to control and breaking bad habits. Your friends advice was correct, when he pauses at the water and starts to show signs of rolling pull back on the reins as a sign of control, then release and spur him slightly just as you would to make him go. If he hesitates spur him a little harder and direct his head with the reins. If he tries to buck again grab the reins and turn his head into his side(horses can not buck when there heads are turned like this) and stay in control until he stops. Then instead of turning back start the entire process of crossing the water again. DO NOT GIVE UP AND TURN AROUND. This will show who is in controll and promote better riding. Hope this helps(: