I think you are two updates late with this topic. These hidden spots were discovered in first week of the update they were released, thanks to obolus’s lure rod that is seen kilometres away
I was about to say these were diving structures, but that doesn’t quite look like one I’ve seen. Pretty cool find, I can vaguely remember one topic mention these as “obulus nests” but it’s otherwise pretty obscure.
Randomized per server, but diving spots do appear to spawn in specific areas made for them. Mostly, they’re along what I’d call the “trade routes” between islands - such as a straightshot from the Shining Plains coast to Wind-Row, or going from Sailor’s Lodge/Fort Talos to Silverhold.
Now, the types that spawn? Those are fully randomized.