Using buckets towards fish underwater can sometimes cause air to appear in the water
Steps to reproduce:
1. Get buckets
2. Find a fish
3. Use bucket
4*. Done
*If not, try a few more times
Related issues
Comments

Had that bug happen to me too, with both empty and filled buckets... I was spam clicking into the water though. Even with source blocks on all 4 sides (ocean), it wouldn't reproduce the water unless I would put it myself back in there
Ah, then I believe this is MC-12363.

dont know if original reporter's issue is the same as mine, but my issue looked similar to MC-12363 indeed. I wasn't turning though. Don't know if it changes anything. 🙂
Are you using empty buckets or water buckets to pick up the fish?