As you enter the door, do a running jump and as you are in mid air use the grapple. Balance a bit and jump to the ledge on the other side of the water. (You will need to use the Down arrow)
There's an artifact in the water if you want to pick it up, it's on the right side next to the other side as you enter the room.
Use the ledges on the left side to get up and jump on the lever. Use space to sit on it and wait until it's fully down. A door will open under water.
Swim through the newly opened door, you'll find an underground door, use the switch in the middle to open .. and get back up to the boat.
A cinematic will follow and yeah, that's the end of the level. I think it will make it into the Guiness record book as the shortest Tomb Raider level ever.