Whilst on the wooden platfrom and facing the Rocket, locate a small triangular ledge (#1) opposite to the left. Drop into the water and swim towards it. Swim as close to it as possible. If you try to climb up to it, you will see that it is impossible: as you press Action, you will see Lara acting strangely and hear small splashes, but this is exactly what we are going to take advantage of. Facing that ledge, look to the left. There is another sloped ledge (#2) there.

Turn to face that sloped ledge and swim close to it, facing its middle part. Make sure Lara is sticking to the underwater part of the ledge and press Jump.

Lara will dive in the water, but because she will collide on solid land underwater, she will try to stand on it. Immediately as you see her trying to stand up, press Jump together with the Down direction arrow (or press only Jump).

If you did everything right, Lara will do a backflip and land just at the corner of the triangular ledge. Now you can turn around and jump on the higher ledge ahead, then to the green rocks a bit to the left and from there to the long green ledge with the Rocket. After you grab the Rocket, drop in the water and climb on the wooden platform.