You begin the level in the ocean, underneath the Sub. You control Lara exactly the same way as you would when she needs to swim. The only exception is that you cannot roll. Do not worry about your breath (you picked up the Aqualung, remember?). However, the only weapon you have is one attached to the back of your suit which can throw Chaff Flares (check your inventory to see them). This can help you to elude your enemies, as they will not be able to see you for a while when you fire one. To use a Chaff Flare, press the Sprint button. Note that you have only 10 of these Flares, though. In any case, try to avoid the enemy altogether, and focus on finding your way forward at the bottom of the sea.

Swim downward to exit the Sub completely and then forward until you reach some rocks. Here you can go to the right or the left-hand side.

Secret (1-1\ total: 16-36) - Go to the right hand side and pass the rocks. There is a small sub in this open area. You can avoid it or use one of the Flares (avoiding it seems easier). As you enter, swim to the back left corner and swim down behind the U-boat. There is an opening down here that leads to a Golden Rose (see this screenshot to get oriented). Get it and return to the Sub, where you started the level, the same way you came. The current may not help this process, but keep pressing the Swim (Jump) button.

Now follow the left-hand side from the beginning. As soon as you enter the new open area, stick to the left-hand wall. You go past a passageway that you cannot follow because the current prevents you. Another small sub is coming from the right. Avoid it and follow the passageway to the left. There is another small sub in the next open area that comes from the left. Immediately swim down through the opening of the U-boat.

Inside the U-boat - When you get inside the wreckage of the U-boat, locate the small opening at the bottom. Swim through it and then through another opening on the left above. Then go through another opening far left and finally follow the last opening, which is on the right.

The cut scene shows Lara entering a small chamber and finding a box. This includes the Spear of Destiny (what Mikhailov and Yarofen were talking about and wanted to find). She opens the box, but just then the ceiling collapses and breaks the Aqualung.

Making your way back - Save your game after the cut scene. You need to get back to the Sub. As the Aqualung has been destroyed, you only have your regular breath in which to make it. If you were not hit by the small subs (your health is 100% full), and you are quick enough from this point forward, you can make it to the Sub. Otherwise you may need to take a Medipack. When the cut scene ends, the exit is straight ahead. Exit; the next opening is on the left. The next one is on the right, and the last one is at the bottom. Swim upwards to exit the U-boat.

You can return following the same way you came or by making a turn to the left and following another passageway altogether. The current will push you close to the Sub. It is the passageway you could not follow before because the current prevented you. Swim towards the Sub, then underneath it from where you exited it from at the beginning of the level.