I started a new game. I play as a cipher who kills everything.

 

So I managed to kill the caravan on hard (unfortunately, not on PotD; I was like 1 hit from doing it and then lost again). Because of that, I couldn't pick up the waterskin and that is why I cannot deal with the ooze on the wall in ciliant lis ruin. Is there any place I can buy it or find it?

I doubt it somehow. It's a key item after all.

 

Still, that ooze is hardly a big deal right? With such an extreme play-style, I think you'll run into a few minor problems. 

It's more about being able to access more content. I find it silly to forget about the treasure because of... well, something dirty on the wall? Especially when I just read about it in the journal that I'd grabbed from the dead body nearby. I'd like to see an option to buy some waterskins if needed, just to complete the quest with ooze later.

 

Anyway, I used console to add water to my invetory. Problem solved.