When you approach the wide landing at the top of the stairs...

...an ornamented skeleton rises out of the floor. Either move past it, destroy it with explosives, or use the shotgun to knock it into the rectangular hole between the two statues. Drop through that hole into the pool below and swim down and forward.

All 3 square doors open if you swim close to them and press Action, but there is nothing behind the left or middle door.

Open the door on the right (north)...

...and swim through the passageway...

... to a room where you can surface.

As you enter the next room, a living golden uraeus emerges from the stairway on the left (south). If you lock onto it right away and continue firing with pistols, you may be able to shoot it down before begins firing energy bolts at Lara. Of course, Uzis or explosive arrows work even faster.

You'll know it's about to fire a blast when sparks start swirling around it. If that happens, side flip out of the way...

...to avoid the blasts and continue shooting.

If it dive-bombs Lara, try and roll/hop out of its way so it doesn't jab her with its stinger-like tail. The tail does quite a bit of damage and can also poison Lara. If this happens, the health bar tuns yellow and you'll need to use a medipack to antidote the poison. Keep shooting until the uraeus falls out of the air.

Crouch and shoot the 4 hollow benches to get some goodies: small and large medipacks, plus 3 boxes of regular shotgun shells and 3 boxes of wideshot shells. You can now explore the areas at the top of the three staircases in any order you like.

[Pharos Walkthrough]