Uh, I think this module is severly underbaked, and a lot of information is missing.
How did the cult enslave the drowned god? Is there any way to free it? How do they recruit people?
Also, worst of all, there's no mention about where the Drowned God is and how it can be encountered. Despite having a stat block it is not stated to be in the Drowned Temple or any other location, and it's not in the encounters table.