Have you considered giving the Water it's own initiative (maybe the simpest option would be at the start or the end of the round). Then everyone moves at the exact same time, and the goal is to avoid getting slid into an area you don't want to end in, and bumping into each other. I would probably still combine it with a Fortitude Attack, a hit results in sliding 2 squares, a miss in sliding 1 square. Athletics/Acrobatics might be used to negate the attack, reduce (or increase, with control?) the distance by 1 square, or give a bonus to your Fortitude defense.