Even if we somehow magically cut out the turn and the skiier, instantly, was facing the opposite direction and going at the same speed on a surface of water that was moving with him instead of against him.... he would sink. This was the bases to Einstein's theory of relativity. lets take it a step further and say he wasn't moving at all compared to the rushing water. he would sink. Yet from the perspective of someone standing on the bank of the river, he is moving quite fast with the river. According to you he's moving so he should still be skiing.