What constitutes a fatal fall into one block deep water?

Solution 1:

If you put a fence at the base of the fall, mobs will still take their regular falling damage. Due to the fence taking up 1.5 blocks of space (this is why you can't jump over fences), the mob will hit the fence instead of the water breaking the fall.

This mechanic can be applied like this (not my trap - it's part of a world I downloaded):

Mobs spawn in this gigantic tower...

...and hit these fences. The water then funnels the items into the middle (which then connects to another stream flowing the lava you can see, to kill any mobs that fell through the middle).

Solution 2:

The minecraft wiki page on water states:

[T]he highest number of block you can fall off of without hurting yourself is 3 ... You can hold the jump key to land in water only 1 block deep if necessary.

So considering the inverse, a single block of water will only prevent fall damage if you're holding in the jump key. Since mobs have no reason to jump if there are no players around, a 23-block drop into 1 block of water should be enough to kill any mob instantly. Just make sure you're nowhere near your trap when the mobs fall, so they're not trying to jump as the descend.