I created a mob fall trap over the deep ocean with all caves lit well within 128 blocks every direction (actually considerably more) with the first spawn platform starting approximately 37 blocks above ocean level. This is a design I have used for many years on many different versions of MC from PC, MCPE, Xbox 360 and One with great success. It is 20x20 with 4 spawning platforms in the corners with a 4x4 drop chute.The water streams are 2x8 from the 4 directions with additional water source in the corners of the spawn platforms. This is the basic layout. I have noticed that mobs spawn like crazy (even while I was on the opposite side while building UNTIL I ENCLOSED THE SPAWN CHAMBER . It would appear that for mobs to spawn they need open sky since on the roof of the structure the mobs spawn like crazy until I properly lit the roof to prevent spawning on the roof. The chambers are completely dark by the way and the structure is made with cobblestone.

I forgot to mention that this is a large structure designed to resemble an oil platform. The fall trap is adjacent to a fully functioning Golem farm that contains approximately 22 transported and bred villagers made completely in survival. This is a massive structure that is only fractionally built.