No.  You can use whatever standard swim technique you want.  You are even permitted to invent one of your own.  It really does not matter so long as you go from the start, follow the course buoys, and finish at the end before the cut-off time.  It is really that simple.  The crawl (or freestyle) is used because it is the most efficient stroke that conserves the most energy while allowing the swimmer to glide swiftly through the water.