The goal in the room is to get a floating wooden platform next to a ledge that leads to the small chamber where the Key of Poseidon lays on a pedestal. However, the platform is trapped under a floor grate in the very bottom of the room. Lara has to raise the water level first, so that she's able to pull the underwater lever that's attached to the center column. This is done by climbing up using the ledges on the column and pulling a movable block away from the fish-shaped fountain. This raises the water, but before going to pull the underwater lever, Lara should push another movable block over the ledge to the middle section of the room. The block is located opposite the fish fountain.
Now she can go and pull the underwater lever on the column. This releases the wooden platform, but it gets stuck under a walkway. Lara has to go back up, lower the water level by pushing the block to the fish fountain on the upper part of the room, an then jump to the water thet's now on the middle part of the room. The block she pushed over the ledge earlier is now near another fish fountain. She has to use the block to lower the water level even more. When the water is almost darined out from the room, Lara can climb down to the bottom of the room and use her grapple to move the wooden platform to the other grate on the floor. Actually, the platform should be moved to even further, but the rubble on the floor is on the way.
It's time to raise the water level by climbing back up to the middle part of the room and removing the block from the fountain. By using her grapple again, Lara moves the platform to a good spot where it helps her to reach the Key of Poseidon. Raising the water level again at the upper part of the room makes it possible to enter the little chamber.
An overview of the middle section of the Poseidon Room