When transitioning from defensive to aggressive swimming, simply roll from your back to your belly.  Make certain you keep your feet on the surface to avoid foot entrapment.  Aggressive swimming works best in deeper water, over 2' in depth.  Swimming in pushy water is very tiring, especially if the water is cold.  Plan on using short sprints of aggressive swimming to reposition or get to a close by objective.  Take breaks by flipping back to defensive swimming.  Be very careful when you reach shallow water.  It will be extremely tempting to stand - make certain the water is shallow enough and take your time.