I recommend the water version for a 100% chance of keeping Endermen from escaping the trap. The farm also works very well without water, but if you stand there for a long time, Endermen may build up outside and reduce the farm’s efficiency.

If you don’t want water on the roof, place slabs there instead of full blocks to prevent Endermen from spawning on top.

Note: If you paste this farm into your world, make sure entities are enabled and the difficulty is not set to Peaceful, or the endermite will despawn.