Hi all, i made a spawning platform for a trident farm, it's basically on a river biome, Y 230 (since there is no Y limit on a river biome), no light, more than the 2 deep water required, I am 24 blocks + away from the farm, and obviously the mob cap is 0 since i am afk at 230 +, so why aren't any drowned spawning? Tested this on a server and singleplayer with the same results.
There are already many problems that I can see. These are common mistakes but sometimes these "rules" that Minecraft has are inevitable.
The higher a block is the lesser chance a mob has of spawning. For example at y=5, a mob has a 1/5 chance of initiating a spawn. It's don't quite know how to define "initiating" but just know that the higher a farm is the worse the farm will be.
The amount of spawning locations, if there aren't enough spawning locations, the farm will obviously be slow. Each block has their own individual chance to spawn a mob (it doesn't technically work like this but you can think about it this way). Most times, when battling spawn chance people simply increase the amount of spawning spaces so mobs have more chances to spawn.
You can try to fix it by building it closer to the ground. Find a river biome around a flat around of around Y= 60 - 70. Build exactly 128 blocks from the highest point in that area and that will be where your AFK platform. Build your farm around that area and make it large, mobs at that Y level will have a hard time spawning so compensate with a bigger farm.
I understand it's hard to make a large farm in such a small biome like a river so I suggest to try prioritizing a large lake over a nice flat area and try to excavate any land that might be in the spawning radius of you (Radius 128 block sphere, centered around you).
Try making multiple layers below you and funnel them towards your afk spot with soulsand elevators and villagers/turtle eggs to lure them towards it.
Half the battle in making a good farm is finding and creating a good environment, Good luck.