I can confirm this too on 19w14b. However, prior to the latest snapshot (so on 14a I think) I did have Drowned occasionally. I haven't played enough on this snapshot to know for certain how often I find them. There's definitely no fish though which is super frustrating.
This just happened to me! I've only tamed black cats so far, and just now tamed a horse which game me that advancement. I'm on 18w49a.
I hope they fix this soon. I'm on the most recent snapshot 18w48b and it's so slow. I'm not teleporting or switching dimensions, I'm just wandering on foot and the horizon all around is just empty. When I get closer, I have to wait a few minutes for more world to show up. The game in general is also extremely slow and laggy. The only time this doesn't happen is if I'm digging underground. F3+A doesn't help, it just reloads what I already can see and doesn't seem to speed up the loading of the rest.
I'm having the same problem here. Closing the world and reloading it helps for the chunk I'm in and some nearby, but the problem still persists elsewhere. It's getting really frustrating because not only can I not see, but mobs will spawn like crazy in these dark patches even in broad daylight. F3+A to reload the chunks does not fix it. I'm on the new Pre-Release update.