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.