Hit a tree and land your body in the water -- pretty simple.
Load up the second trail on the first mountain (Graterhorn - Wandering Woods), and right after the second checkpoint is a big lake surrounded by trees. There is a nice cluster of trees right up against the water along the first curve after the checkpoint. Line yourself up, floor it into a tree (RT + A), and you should fly into the water. If you miss, the checkpoint is right there so it's a fast reset.