Good. I'd assume that the mob will walk off the edge and into water if the fall won't hurt them, and I doubt they are smart enough to "know" that the very water they are jumping into will push them off a ledge to their plummeting doom. So, time to redesign my grinder then.
It's a shame though, that defeats the entire point of the vertical fall damage grinder, which was killing speed. Now the time to kill a mob will be much greater. Oh well. Maybe I'll have to add more vertical spawning floors (but smaller floors) to counteract that. OH MY GOD, grinders can now go up to lvl 256! Maybe it's time for a hugely tall grinder....
Well, those kind of grinders are only slower when it starts. But once you ennemies are starting to drown, it gets as fast as it can be. + you get 100% loot tks to the drowning