If you are building a mob grinder (not powered by a spawner but by natural mob spawning) right above the ocean, and the very bottom of the grinder is at y = 63 (the top of the water), what is the best y level to afk at? I remember reading somewhere that mobs will only spawn with a certain range of the player, but I don't remember exactly what it was. I don't want things like drowned to spawn in the water, because it will count towards the mob cap and decrease the amount of mobs that spawn in the farm. So, how high up should you be so that the mobs will spawn in the farm, but not in the ocean?
"While mobs spawn in a 15x15 square of chunks centered on the player, they immediately despawn if they are not within 128 blocks from the nearest player, meaning mob spawning is effectively within a 128 block radius sphere around the player.
In 1.2.1, the max build height was raised from 127 to 255. This made building farms way up above the previous height limit, or just having the player AFK there, an attractive option. For example, if you built a mob farm in the sky at around y=90 and AFKed at y=200 or above over a relatively flat area, then the only mobs that would (effectively) spawn would be those in your mob farm, since your farm would be within 128 blocks but the mobs on the surface at about y=64 and below in unlit caves would be more than 128 blocks away and not affect the spawning rates. "
This means you would need to AFK at 63 + 128 = Y 191 so mobs will not spawn under Y 63.