If we’re talking aquatic wights (specifically oceanic), the only legitimate example I can think of would be Patchface, right? It lends a sort of irony that the two arguable examples of Drowned God-esque magic — Patchface and Euron’s incoming sacrifie — are done by/to non-believers.

Patchface certainly counts. 

As for the believer/non-believer thing, I think it depends on how you view “gods.” Are they entities for whom worship is food, or are they entities at a scale so large that our worship doesn’t really concern them? If the Drowned God follows Cthulhu in any way, it’s the latter. 

So it doesn’t really matter whether Patchface was a believer or not, he gazed upon the face of the Drowned God and for a merest fraction of a second made eye contact with it, and the psychic force of that brush with divinity broke his mind into a kaleidoscope (collidoscope?), turned him into a prophet, and then brought him back from the dead.