This sequence is indicated by the solid green line on the diagram in the main walkthrough.
After using the switch, stand at the tunnel opening facing east (switch behind Lara).
Jump into the water and swim down then forward (east)...
...then left (north). So far there are no choices; you're just following the tunnel.
At the first intersection you can go left (west) or straight (north). Go left.
The tune slopes upward, and you'll pass a tile with a snake on it and a passage leading straight up. Don't go up; instead continue on straight (west). Follow the tunnel as it slopes down...
...then turns right (north).
Approach the tile with the hand symbol and swim straight up.
Ignore the tunnel leading off to the right (east) and continue upward to surface in ROOM #2.
Pull the switch to open the trapdoor to the next room (#3 in the diagram).
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