While I was playing my world on the new snapshot version, I want to go swim for fish to tame a cat, but I noticed that there were no fish spawning in the river. I also had this problem looking for squids earlier on. Curious, I made a new world and went to go see if any fish spawned anywhere, and they didn't. The only mob that did spawn were elder guardians in sea temples. You can see more in the attached files. This seems to only be an issue with the snapshot, as in the newest official release, they spawn just fine.

I couldn't fit the video into the attachment section, so here's a Google Drive link: https://drive.google.com/file/d/1riG_4VQjfQWS5TH3EXH15tfw1JwjjNAq/view?usp=sharing