Ferrying while swimming is also the same technique you use when boating.  Keep your head up so you can see where you are going, set a ferry angle somewhere around 45^o to balance your upstream swimming against the downstream current and swim hard.  Faster water uses a smaller angle and very slow water you can simply swim directly across at a 90^o angle.  Some advocate a mad dash perpendicular to the river.  This may be effective for a very short dash but exposes the full body to the current and will quickly sweep you downstream.  The ferry angle is the only way to go when crossing a wide stream with current.