You'll want to use a particle system that follows the rules of gravity and possibly wind, if you want. It'd look something like this...

Draw a water drop and make it a movieclip. Have it set to "Export for Actionscript" with the linkage name "water".

Code:
density = 2;
gravity = 1;
wind = 2;
createEmptyMovieClip("sprinkler", 0);
sprinkler.depth = 0;
sprinkler.onEnterFrame = function() {
   for (var i=0; i<density; i++) {
      var clip = this.attachMovie("water", "water"+this.depth, this.depth++);
      clip._x = Stage.width/2; // x origin for water
      clip._y = Stage.height-100; // y origin for water
      clip.xspeed = Math.random()*6-3; // initial velocity horizontal
      clip.yspeed = -Math.random()*15; // initial velocity upwards
      clip.onEnterFrame = function() {
         this._x += this.xspeed;
         this._y += this.yspeed;
         this.xspeed += wind;
         this.yspeed += gravity;
         if (this._x < 0 || this._x > Stage.width || this._y > Stage.height-100) {
            removeMovieClip(this);
         }
      };
   }
};
This should spray water drops everywhere. I haven't tested it so let me know if it doesn't work.

If you want the drops to fluctuate (directed spray at an oscillating angle), I would use a sine wave. Lemme know if you need help with that.