There are no ancient cities in the Deep Dark. The Warden carries what they held: one or two echo shards every time, a one in four chance of a Swift Sneak book, and a one in three chance of a disc fragment.
You do not loot the Deep Dark. You beat the thing that lives in it, or you leave.
Elder guardians in open water
About one in every 250 guardians in deep water becomes an elder. They are spaced out, so you never meet a group of them, and they only appear deep down or in dark flooded caves.
Deep water is not safe just because there is no monument in it.