I cannot get drowned to spawn in the river biomes no matter what I do. I first just searched around and would never find them naturally spawning in the rivers. I then darkened the areas to improve spawn rates, and that didn't work. I proceeded to make a super flat world that was 100% a river biome and created a large structure which I had used in 1.13 to spawn drowned in, and despite the mob cap being zero and the light level being under 7 for the entire 100x50x100 pool of water, no drowned would ever spawn. Therefore I believe this is some sort of bug keeping them from spawning in river biomes, unless I am mistaken for some reason. I also hope this isn't some secret feature, because I really miss having drowned spawn in rivers and come out to attack me, it made the night dynamic very interesting.