I give up; how do you do it?

Haven't you been paying attention to Farli's hints ? ;)
To drain the sewers you need to to find two cylinders and insert them in the wall slots back up on the previous level of the ruins. The sewers are divided in two sections by a wall, so find the first cylinder and go back up to the ruins and place it in one of the wall slots to remove the dividing wall, come back down to the sewers, find the second cylinder in the section that opened up, go back up to the ruins and place it in the other wall slot to drain the water.

Namur is basically correct, but it turns out that you don't have to move the wall first (at least I don't think so). There are two entrances to the sewers (i.e. two stairways leading down) and you can find one cylinder in each of them. The two sections are separated by the moving wall, so you COULD move the wall first but I would recommend just using the two entrances to find the two cylinders. You will probably want to find the two cylinders as quickly as you can, without spending too much time down there while it's still full of water.

I was under the impression that both stairways from the ruins went down to the same sewer section, but i played through it just once so there's a good chance i'm remembering it wrong.

Thanks for the help. :)
For those of you wondering how to find the cylinders, walk around in the sewers until you hear Drake say "OW! My foot!", which signals he's stubbed his foot on one of the cylinders. Back up one square and scan the "OW! My foot!" square with your mouse cursor to locate the cylinder and place it in your inventory.

I've also only played once so I could also be misremembering. Greystone, since you seem to have found the cylinders, did you need to open the wall to get them both?

I don't have the game installed right now, but i took a look at the maps in one of the stickyes and i think you are right.

Waltorious; yes, the first cylinder I found was in the sewer section that's accessible before opening the wall. It was located somewhere in that section's western area. Unfortunately, it didn't occur to me to record the co-ordinates... The second one's in the section beyond the wall, and I can't recall the general whereabouts; I just blundered along until I found it.

You don't have to move the wall to get both cylinders. You just need to go down
the second set of stairs leading to the sewers.

The sewers are divided in half, one set of stairs leading to each half.
One cylinder in each half.

And where i have to use the second cylinder? Exactly? I found two cylinders, but only one button to use one of them.

I found no other button and that obviously cant be used on same switch twice, not with left or rightmousebutton, white or black dot, nothing happens.
https://i.postimg.cc/mrSZj8tv/image.png