This is the hardest of the "hit these things when you crash" achievement, since you need to hit (in any order) a rock, a tree, and a cactus and then your body needs to end up in the water. Cactus only appear on the 3rd mountain, and there are few places with both cactus and water.
Fortunately, there's a really nice spot to do this on the first trail of the third mountain. Get to the final checkpoint before the finish line (Section 7/7), and when you turn left down the hill, you should see a rock, tree, and cactus all clumped up by the river. I just boosted into the rock (
) to have enough speed to launch my body into all the things and go swimming. It may take a few tries, but the achievement unlocks as soon as you manage to do it.
Words are hard, so here's a gif