Whenever I set sail in the ocean, I see a mineshaft generating in deep ocean. Seeing the sight of a floating structure kinda makes me mildly infuriated, because they're not supposed to spawn that way.

You should set "Maximum y-coordinate" value to <ocean_floor_level - 16>, so mineshafts (that can be up to 16 block "tall") no longer appear above the ocean floor, especially if you use Tectonic. I've set it to -16 and haven't found any floating mineshafts ever since.