Fall damage is (blocks-3)/2. So a 23 block drop (or more) should kill you (or a mob). But if I'm building a falling trap and want to use water to collect the items dropped, how much higher does it have to be? The floor would have water running over it to gather the items and monsters would fall to that floor. How high a drop do I need?
The minecraft wiki page on water states:
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.
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).