The tomb entrance is in the lower part of the valley and the entrance is across the cave that you find in the destroyed part of the forest. Right after you enter, prepare for a bear attack. After you kill it, make an outcrop in the wall and swim underwater.
At the other side (the screenshot), climb onto the root and follow it slowly, in order to avoid water flowing out of the pipes. The pattern is simple here - the water comes either simultaneously from pipes one and three, or from the middle one. Wait by the middle pipe while it is inactive.
Right afterwards, you get into a camp. Pick up the Relic by the campfire, and another one up ahead, by the sculpture under the stairs. Climb up (in the corridor to your right, there is a Document) and go left among roots to enter the main part of the tomb.
Jump down and swim up to the boat. Jump into it and attach rope to the beam under the spot that you took the dive in. get to the center in the boat and turn right, where you spot another beam. Attach the rope and get there, thanks to which you can jump on the wall right next to you and enter the next chamber.
Get closer to the mechanism on the wall and give it a pull. This drains water from the chamber and allows you to attach rope to the turning wheel in the water. Then, give the mechanism another pull and get to the turning wheel quickly. As long is water level is low, it is possible to give it a turn, which leaves the mechanism in the appropriate position. Do this quickly, before the chamber is flooded again.
Return to the first chamber (you can get there across the small window) and attach the boat to the beam that you have not yet been using - it is opposite the spot, where you left the boat. Approach the end (these is an archivist map on the platform, and dive in the spot shown in the screenshot. At the other side, grab the mechanism and wait for the water to be drained from the chamber. Finally, launch the rope towards the boat, in order to extend it between the boat and the mechanism. Finally, return to the boat where you need to use the turning wheel to secure the rope in place. This will drain the water from the chamber. There are explosive barrels by the main gate - fire at them and read the book and, on your way back, collect the Document (by the statue in the unlocked corridor).
Reward: Ability "Iron Grip" (faster climbing over rocks and ice).