Passive mob spawning in water top of half slap in my Xbox one world

I was trying to make a passive mob farm in my world i clear all the land around the farm and start to flood rest of the land but there is a problem mob keep spawning in water even after I killed most of them they still spawning in water (even there is lots of grass land available to spawn). I checked thousands of time they not wondering to water they spawning in water even itz 10 block deep. I copy the world in creative fill the area with half slap but they spawning on top of the half slap too. I spend hours and hours to clear mountain and trees around. Please fix this issue.