Now that you are out of your boat, let's work on your fine back stroke technique.  Here is a list of tips for white water swimming:

• If possible, hold onto your boat and paddle.  This speeds rescue efforts and avoids lost equipment - AKA: The Yard Sale effect.
• Stay on the upstream end of your boat.  You don't want to be between a rock and a boat full of water.
• Don't stand up in swift water.  People have gotten their feet stuck in rocks that way.
• Lay flat on your back and use your one free hand scissor kicks to swim to shore.
• If some paddlers come to rescue you, do what they instruct you to do.  They can see a great deal better than you can because they are sitting higher.
• If you are coming up to a dangerous obstacle, for example a strainer or tall drop - ditch the gear and high tail it to safety.  Boats can be replaced, you can't.
• If you have to swim over a tall ledge or through a big hole, tuck into a tight ball to protect your torso and extremities and open up after the drop.
• Keep an eye out for a throw rope and grab it if you can.  The rope should be positioned on your tummy side and over the shoulder on the side they are pulling from.  This helps prevent the rope from going around your neck and keeps your face above water where you can breathe.

The following excellent article from Performance Video gives even more tips and is well worth reading: Swimming in Whitewater.

Some other considerations, if you notice a nearby and substantial eddy by all means use it.  Sometimes you can push your boat across the eddy line faster than swimming across with your boat.  Now that you are free from the boat, you can quickly swim in afterwards.  If a shore is close enough, I like to toss the paddle like a javelin onto the shore - one less item to get in the way and worry about.  Keep your boat upside down so it retains an air pocket and provides you some extra flotation.