Since the last snapshot, 19w13a & b, fishes and guardians seems to no longer spawn in your upgraded worlds and newly created worlds. Squids and dolphins spawn normally though. (This is NOT the case of the Render Distance issue of hostile mobs spawning)
Drowned don't seem to spawn either when there is a deep ocean biome. However, they do spawn normally in the underwater ruins.
Though Elder Guardians spawn normally in their Monument, but the guardians do not, even if your difficulty is on easy, normal, hard or hardcore mode.
And with fishes, the Advancement: "Tactial Fishing" seems to not to be obtainable in survival, unless from Wandering Traders and Fisherman villager.
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Confirmed that there are STILL no fish, anywhere, in 19w14a.
There are also no regular Guardians anywhere, even after spending 15 minutes swimming around an Ocean Monument.
It took a few minutes for an Elder Guardian to appear, but it suddenly did...sort of. It actually tried three times (spawning and disappearing; spawning and disappearing; spawning and disappearing) before it finally appeared long enough to hit me with Mining Fatique. Then it disappeared again...or so I thought.
What actually happened, was that it faded into the wall that was right beside it, and got stuck. It didn't die. It just couldn't get out of the wall, and it could no longer locate me or hurt me. I was able to kill it by hitting the parts that were sticking out of the wall (i.e. its tail parts). During that entire time, there were still no regular Guardians.
One thing I've noted besides the lack of fish and Guardians, is that there are an OVER-abundance of Squid, who are killing themselves in huge numbers on all of the shorelines.
This is also happening to me in the current snapshot, 19w14a.