Guardians in ocean temples don't spawn at all. The elders spawn, but not the guardians. Is it just that I'm on the super-flat preset "Water-world"? Or is it something else? I've tried everything, almost, there's probably still things out there. I am using mcedit, I used it to make almost everything.
Guardians spawn less often in ocean open to the sky than they do in covered areas (i.e. inside the monument). Specifically, spawning will fail 95% of the time if the spawning water block is below sea level (Y=63 by default), all blocks between the spawning water block and sea level are liquid or fully transparent, and the block at sea level has a view of the sky. Above sea level, spawning will fail 95% of the time if the spawning water block has a view of the sky.
I don't know if your world meets those conditions as a super-flat preset. If it meets all those conditions, then you might try to edit your world by deleting chunk of the map and generating them again by going to those location and see if anything as change.
You might have your gamemode on
Peaceful, because then only animals like cows and pigs will spawn, and not hostile mobs.