Minecraft – What constitutes a fatal fall into one block deep water

I highly recommend Etho's excellent subterranean spawning pads with a water-based mobevator for moving mobs to the surface. An ammended tutorial for the mob-spawner and transporter can be found here.

The video doesn't include a mob-killing mechanism but there are multiple examples on his youtube channel. Personally I prefer transporting them a bit to the side of the mobevator, then leading them up a 2x2x4 tube of water with signs creating a gap one tile up from the ground. The mobs swim to the top and drown in the enclosed 2x2x2 cube of water, while their drops fall through the gap and into the bottom 2x2x1 water stream that you can channel wherever you want to.

[Edit: I forgot to address your requirements.]

Easy to build? It requires nothing but pickaxes and a bucket. Glass makes it easier to construct but isn't necessary.

Completely reliable? I have never seen a clog, ever, nor has it ever failed to kill a mob.

Good item-gathering? 100% recovered drops, no lava or cacti to destroy them and you can channel them to a single tile anywhere you want with the output water stream.

The rules for "normal" spawns do not seem to apply fully to Mob Spawners. A dungeon with a completely flooded floor is not safe. Apparently the random block in the 9x9x9 cube can be air.

Here's a flooded dungeon, and here's the same dungeon after I turned the difficulty to Easy (and removed the torch). The spider spawned in the water and climbed up to the ceiling.

Edit: Test repeated with source blocks, and a torch for better visibility.

Setup

Result: still not safe