Welp. I couldn't help myself.
I mean, that depends on what you think a "simple fisherman" is. Specifically, you presuppose that "simple fishermen" are ignorant of all those things, that they are "previously unknown".
One group I play with is going through Rime of the Frostmaiden - the action takes place in an area where the local food economy is largely based... on fish. There's a preternatural darkness over the entire area that they all know is caused by a deity. And every single town has some magical weirdness going on. While they may not be aware of some particular entity you speak of, they know gods and aberrations and fey exist.
The point I was trying to make (albeit poorly) and the point which I think the presented definition of a deity is getting into, is these interactions with otherworldly beings elevates consciousness beyond mundane concerns. The knowledge that something like an aboleth exists leads to the understanding that there is more to the world than mundane existence, and that's incredible/terrifying.
The existence of gods, spirits, fey, fiends, etc. reveals that their is more to the world than what is immediate obvious, therein expanding consciousness of said world. Knowledge of the Fey leads to knowledge of the Feywild, which could lead to knowledge of the Shadowfell, and then the Great Wheel, and then everything else.
Looking up at the stars you remember that each one is a celestial body similar to our own Sun, with planets like our own, and there's more of them than you can even comprehend. Consciousness is expanded. It's the same for knowing a god, a powerful being of some sort, is messing with your weather, and you need to sate its anger somehow.
Tolkien didnt, and according to him, neither did the enlighted of middle earth.
Do you consider Angels. Jinn and so forth to be Deities?
From what I understand, Tolkien waffled regarding whether the Ainur were gods, eventually settling on them as angelic beings, godlike but not worshipped.
I would consider Angels to be deities. Saints too since I'm feeling cheeky. They're polytheisms in an ostensibly monotheistic religion. But further discussion on this risks creating a theological debate that I'm in no way qualified to participate in (I'm not even qualified for
this discussion).