Once you pass through the gate and whilst Lara goes down the steep part, press the left direction key to turn the kayak towards the left hand wall.

Keep pressing left, until Lara reaches the end of the second steep part. Immediately start back-paddling.

Back paddle even if the kayak reaches the end of the rocks. The important thing is not to totally pass them. The water beyond the rocks is so strong and even if you hardly back paddle, you will not make it (or at least it will be very difficult).

Lara will most probably find herself as the screenshot shows. Whether she is like that or not, do not stop back paddling. It is very important not to stop at all, because Lara is very close to the part of the water that is strong (beyond the rocks).

Back paddle and the kayak will move into an alcove. Do not stop and the purple wall (on the right hand side of the screenshot) will push the kayak forwards, but it will also turn it a bit to the left.

Back paddle and Lara will start passing the wall...

... and finally find herself as the screnshot shows.

Keep back paddling to move away from the current that is close to the rocks. Then you can control the kayak easier. The Crystal is on the right hand side of the creenshot and into an alcove. Carefully turn right...

... enter the alcove and claim the Crystal, which is behind the waterfall.