The OE sewers use terrain_bc_scum_01 to simulate a layer of muck at the bottom of the water, visually speaking this works well enough (assuming the player isn't using some fucky texture replacer) but it does cause a slight gameplay issue.
The problem here is that the player can't loot any bodies or pick up any items that drop below the scum layer, which happens quite easily in areas where there is a lot of room between the scum and the actual floor like in "Old Ebonheart, Sewers: Central"
Currently this isn't that big of a deal, but it might become an issue if the player decides to kill Lord Rathine during Cicero's Legion of Shadow questline sometime in the future.