I shot my sons High school W.P. games for 4 years. No better lens than the 100-400, you'll find that switching lens is not practical the ball moves from goal to goal too quickly. I used a monopod.
Best position for action is behind the opposite teams goal, that way you get the faces you care about driving toward you....the back of players heads with no ball in sight (dribbling) is about as compelling as photos of birds flying away from you (ya, I got a million of those).
CP for the daylight glare and pretty fast shutter, as already mentioned. You want to freeze the water movement. Water will be your worst enemy, darn stuff is everywhere and 9 times out of 10 it will ruin your best action shots.