How does the Cistern work?

[Black Rabbit]

maybe it's just me or a bug in my game but there was no lurker in the cistern (or organic stiff) and if it was well..it was blind because I was in water for 5 minutes splashing and nothing hit me xD I suppose all of the splashing is a big mindfuck (pardon the expression) but it worked well...anyone would get suspicious after encountering the first water horror